feat(marketplace): inherit description/version from local-path apm.yml

[imbabamba]

TL;DR

apm pack now auto-fills description and version in
marketplace.json from each local-path package's own apm.yml --
the same fallback the remote source path has used since #1061.
A curator-side value under marketplace.packages still wins when
set, and the local read is path-traversal-guarded against the
project root. Validated against the full unit suite (15837 tests)
with 13 new tests covering the local fallback path.

Problem (WHY)

The remote branch in ClaudeMarketplaceMapper.compose already reads
description and version from each resolved package's own
apm.yml when the curator entry omits them; _fetch_remote_metadata
populates that lookup over HTTPS.

The local branch had no such fallback. It emitted only what the
curator wrote in root apm.yml's marketplace.packages[]. A stale
comment in _prefetch_metadata even claimed "Local-path packages are
skipped (they carry their own metadata)" -- but the local code path
never read any metadata at all.

For a monorepo marketplace using source: ./plugins/<name> entries,
that gap leaves producers with two options:

• Duplicate the description between root apm.yml's
  marketplace.packages[] and each plugins/<name>/apm.yml. The
  two copies drift over time -- the producer edits one and forgets
  the other.
• Run custom sync tooling outside apm pack to keep them aligned.

Remote sources do not have this gap. This PR closes it for local
sources by mirroring the existing remote behavior exactly.

Approach (WHAT)

Layer	Before	After
builder._prefetch_metadata	Filtered out local packages. Returned {} under --offline.	Reads local packages from disk first (always), then remote concurrently (still skipped under --offline). Returns one dict the mapper consumes the same way regardless of source kind.
builder._fetch_local_metadata	Did not exist.	New method. Reads <project_root>/<subdir>/apm.yml, extracts description and version. Path-traversal-guarded via ensure_path_within. Skips a source that resolves to the project root itself.
output_mappers.ClaudeMarketplaceMapper.compose (is_local branch)	Emitted curator-side fields only. No fallback.	Same hierarchy as the remote branch: curator wins; otherwise fall back to package-manifest meta. Verbose diagnostic on divergence.
docs/reference/manifest-schema.md	description row read "Pass-through to marketplace.json" with no mention of the fallback.	One-paragraph note after the marketplace.packages table covers the fallback for both source kinds.

Remote-source override semantics are unchanged, including the
verbose diagnostic that logs divergence between curator and package
manifest. Local sources log the equivalent diagnostic with
(package: ...) instead of (remote: ...).

Implementation (HOW)

File	What changed
src/apm_cli/marketplace/builder.py	New _fetch_local_metadata(pkg), mirroring the _fetch_remote_metadata shape -- reads pkg.subdir from disk, validates path containment, returns dict or None. _prefetch_metadata rewritten to process local packages serially first (no thread pool needed), then remote concurrently. Stale "Local-path packages are skipped" comment removed.
src/apm_cli/marketplace/output_mappers.py	is_local branch in ClaudeMarketplaceMapper.compose reads meta = remote_metadata.get(pkg.name, {}) and applies the same curator-wins-else-meta hierarchy as the remote branch. Verbose diagnostic on divergence.
tests/unit/marketplace/test_builder.py	New TestFetchLocalMetadata class -- 8 tests: happy path (description + version), description-only, missing apm.yml, missing subdir, path-escape, project-root edge, malformed YAML, empty subdir field.
tests/unit/marketplace/test_local_path_compose.py	5 new compose-level tests: description inherited from package manifest, version inherited, curator override wins, missing per-package apm.yml omits both fields, project-root edge does not read the marketplace's own apm.yml.
docs/src/content/docs/reference/manifest-schema.md	One-paragraph note after the marketplace.packages table covering the fallback rule.

Edge cases handled

• Missing per-package apm.yml: returns None; no key emitted.
• Malformed YAML: returns None; error logged at debug level;
  the build continues.
• Source escapes project root: ensure_path_within raises;
  _fetch_local_metadata catches and returns None.
• Source resolves to the project root itself: explicit check;
  returns None so the marketplace's own apm.yml is never read
  as a package manifest.
• Empty subdir on the ResolvedPackage: defensive early
  return.
• --offline: local reads always run (filesystem, no network);
  remote reads are still skipped.

Validation evidence

$ uv run --extra dev ruff check src/ tests/            # All checks passed!
$ uv run --extra dev ruff format --check src/ tests/   # 1158 files already formatted

Tests: 15837 passed, 1 skipped, 21 xfailed (full tests/unit +
tests/test_console.py, ~1:25). 1418 passed across
tests/unit/marketplace, including the 13 new tests added in this
PR.

Follow-up to #1061

#1061 added the remote-source fallback (_fetch_remote_metadata)
to close the "remote-source pass-through metadata dropped" gap.
The same gap existed for local sources. This PR closes it.

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