Refactor stack management into forest command group with conflict-preserving rebase

This branch consolidates stack operations (split, merge, reroot) under a unified forest command group. Replaces separate consolidate/split commands with forest subcommands that provide better UX and enable conflict-preserving rebase workflows.

## Files Changed

### Added (14 files)
- `src/erk/cli/commands/forest/` - Forest command group (list, show, rename, split, merge, reroot operations)
- `src/erk/core/forest_ops.py` - Core forest operations (forest membership logic, tree queries)
- `src/erk/core/forest_types.py` - Forest dataclasses (ForestMetadata, Forest, ForestWorkTree)
- `src/erk/core/forest_utils.py` - Forest utilities (membership calculation, worktree grouping)
- `tests/commands/forest/` - Forest command tests (split, merge, reroot workflows)
- `tests/core/test_forest_*.py` - Forest operations tests
- `tests/fakes/forest_ops.py` - Fake forest operations for testing

### Deleted (4 files)
- `src/erk/cli/commands/consolidate.py` - Replaced by forest merge command
- `src/erk/cli/commands/split/` - Replaced by forest split command
- `tests/cli/commands/split/` - Test consolidation

### Modified (31 files)
- Git integrations (RealGit, NoopGit, PrintingGit) - New abstract methods for reroot
- CLI (cli.py) - Register forest group, remove old commands
- Create command - Auto forest lifecycle management
- Documentation (glossary.md, erk.md) - Forest terminology, updated references

## Key Changes

- **Forest command group**: Unified interface for split, merge, reroot, list, show, rename operations
- **Conflict-preserving rebase**: Reroot operation saves conflict states in git history with `[CONFLICT]` commit markers
- **Auto forest lifecycle**: Create command now auto-creates or joins forests based on Graphite parent/child relationships
- **Git layer enhancements**: 6 new abstract methods (rebase_branch, get_conflicted_files, commit_with_message, is_rebase_in_progress, abort_rebase, get_commit_sha)
- **Documentation**: Added forest glossary entries and updated integration examples

## Critical Notes

- Old consolidate/split commands removed; forest commands provide complete replacement
- Reroot requires Graphite enabled, clean working directory, and all branches to have worktrees
- Forest names are labels only - renaming never affects worktree paths

Author: schrockn

.claude/skills/erk/references/erk.md

@@ -1127,7 +1127,7 @@ repo = discover_repo_context(ctx, Path.cwd())
 **List worktrees:**
 
 ```python
-worktrees = ctx.git_ops.list_worktrees(repo.root)
+worktrees = ctx.git.list_worktrees(repo.root)
 # Returns: list[WorktreeInfo]
 ```
 
@@ -1176,7 +1176,7 @@ def test_list_worktrees():
     )
     ctx = ErkContext(git_ops=git_ops, ...)
 
-    worktrees = ctx.git_ops.list_worktrees(Path("/repo"))
+    worktrees = ctx.git.list_worktrees(Path("/repo"))
     assert len(worktrees) == 2
 ```
 
@@ -1189,7 +1189,7 @@ Commands behave differently based on current directory:
 repo = discover_repo_context(ctx, Path.cwd())
 
 # Different behavior in root vs worktree
-current_branch = ctx.git_ops.get_current_branch(Path.cwd())
+current_branch = ctx.git.get_current_branch(Path.cwd())
 if current_branch == repo.default_branch:
     # In root worktree
 else:
@@ -1203,7 +1203,7 @@ Check if Graphite is available:
 ```python
 if ctx.global_config and ctx.global_config.use_graphite:
     # Use Graphite features
-    parent = ctx.graphite_ops.get_parent_branch(branch)
+    parent = ctx.graphite.get_parent_branch(branch)
 ```
 
 ### GitHub Integration

docs/agent/glossary.md

@@ -38,6 +38,66 @@ A **managed worktree** created and maintained by the erk tool.
 
 **Example**: `erk create my-feature` creates both a git worktree and an erk.
 
+### Forest
+
+A **named collection of worktrees** belonging to the same Graphite stack.
+
+**Purpose**: Enables unified stack management operations like split, merge, and reroot.
+
+**Key characteristics**:
+
+- Forest names are pure labels stored in `forests.toml` metadata
+- Renaming a forest NEVER moves worktrees or changes filesystem paths
+- Worktree paths remain stable: `~/.erk/repos/<repo>/worktrees/<original-name>/`
+- Membership determined automatically by Graphite stack structure
+
+**Lifecycle**:
+
+- Created automatically when `erk create <name>` called from trunk
+- Joined automatically when `erk create -s <name>` called from forest member
+- Deleted automatically during `erk sync` when empty (zero worktrees)
+
+**Example**: A forest named "auth" might contain worktrees `auth-redesign`, `add-oauth`, and `add-2fa`.
+
+### Forest Membership
+
+Automatic assignment of a worktree to a forest based on Graphite parent/child relationships.
+
+**Rules**:
+
+- A worktree can only belong to one forest at a time
+- If branch's parent is trunk → new forest OR join existing if siblings present
+- If branch's parent is in forest → join that forest
+
+**Example**: Creating branch `add-oauth` from `auth-redesign` automatically adds it to the `auth` forest.
+
+### Empty Forest
+
+A forest with zero existing worktrees, automatically deleted during `erk sync`.
+
+**Detection**: Checked by comparing forest metadata against actual worktree directories.
+
+**Cleanup**: Automatic and silent during sync operations.
+
+### Reroot
+
+Conflict-preserving rebase operation for an entire forest.
+
+**Purpose**: Rebase all branches in a stack while preserving conflict states in git history.
+
+**Benefits**:
+
+- Permanent audit trail of conflicts and resolutions
+- Better PR reviewability
+- Easier debugging of complex merges
+
+**Commit format**:
+
+- `[CONFLICT] Rebase conflicts from <parent> (<sha>)`
+- `[RESOLVED] Fix rebase conflicts from <parent> (<sha>)`
+
+**Example**: `erk forest reroot auth` rebases all branches in the auth forest.
+
 ### Repo Root
 
 The main git repository directory containing `.git/` directory.

src/erk/cli/cli.py

@@ -3,11 +3,11 @@
 from erk.cli.commands.checkout import checkout_cmd
 from erk.cli.commands.completion import completion_group
 from erk.cli.commands.config import config_group
-from erk.cli.commands.consolidate import consolidate_cmd
 from erk.cli.commands.create import create
 from erk.cli.commands.current import current_cmd
 from erk.cli.commands.delete import del_cmd, delete_cmd
 from erk.cli.commands.down import down_cmd
+from erk.cli.commands.forest import forest_group
 from erk.cli.commands.goto import goto_cmd
 from erk.cli.commands.init import init_cmd
 from erk.cli.commands.land_stack import land_stack
@@ -16,7 +16,6 @@
 from erk.cli.commands.prepare_cwd_recovery import prepare_cwd_recovery_cmd
 from erk.cli.commands.rename import rename_cmd
 from erk.cli.commands.shell_integration import hidden_shell_cmd
-from erk.cli.commands.split import split_cmd
 from erk.cli.commands.status import status_cmd
 from erk.cli.commands.sync import sync_cmd
 from erk.cli.commands.up import up_cmd
@@ -37,12 +36,12 @@ def cli(ctx: click.Context) -> None:
 
 # Register all commands
 cli.add_command(completion_group)
-cli.add_command(consolidate_cmd)
 cli.add_command(create)
 cli.add_command(current_cmd)
 cli.add_command(down_cmd)
 cli.add_command(checkout_cmd)
 cli.add_command(checkout_cmd, name="co")  # Alias
+cli.add_command(forest_group)
 cli.add_command(goto_cmd)
 cli.add_command(land_stack)
 cli.add_command(up_cmd)
@@ -55,7 +54,6 @@ def cli(ctx: click.Context) -> None:
 cli.add_command(del_cmd)
 cli.add_command(rename_cmd)
 cli.add_command(config_group)
-cli.add_command(split_cmd)
 cli.add_command(sync_cmd)
 cli.add_command(hidden_shell_cmd)
 cli.add_command(prepare_cwd_recovery_cmd)