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

[schrockn]

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

[schrockn]

• Add forest reroot command and integrate with create/sync #641
• Replace erk consolidate with forest merge command #640
• Replace erk split with forest split command #639
• Add forest query commands (list, show, rename) #635
• Refactor stack management into forest command group with conflict-preserving rebase #634 👈 (View in Graphite)
• master

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