Add project sync --all with machine-local .lnkprojectcache

[polymorcodeus]

Summary

Adds lnk project sync --all to reconcile every stored project scope in one command, backed by a machine-local .lnkprojectcache that maps stored project IDs to their local checkout paths. Replaces the previous $HOME walk in lnk doctor with cache-driven project discovery.

What's new

• lnk project sync --all - Reconciles each stored project that has an available cache entry, carrying through --dry-run, --prune-deletions, and --force.
• .lnkprojectcache - A gitignored, machine-local JSON cache that records each project's ID, local checkout path, and state (available, missing, or not-downloaded).
• Automatic cache maintenance - project push and project sync record or refresh the project's cache entry on success.
• lnk project cache --scan <dir> - Explicitly scans provided directories to populate or repair the cache; repeatable.
• Cache-driven doctor checks - lnk doctor now uses .lnkprojectcache to find orphaned storage, broken project symlinks, and missing checkouts instead of walking $HOME.

Usage

Command	Purpose
lnk project sync --all	Reconcile every cached project
lnk project sync --all --dry-run	Preview reconciliation without changing files
lnk project cache --scan ~/code	Discover local checkouts under ~/code and update the cache
lnk project cache --scan ~/.config/project --scan ~/work	Scan multiple directories

Behavior changes / guardrails

• No default scan roots: project cache --scan requires explicit directories to avoid accidental broad scans of $HOME.
• Hidden directories are skipped during discovery except for .git itself.
• not-downloaded cache entries are treated as intentional and are not reported as orphaned storage.
• Doctor no longer warns about .lnkinclude patterns that match no files; global patterns may intentionally not match in some projects.
• Projects with no cache entry or a missing cache entry are reported by lnk doctor with a suggestion to run project cache --scan.

Testing

• Unit tests for ProjectSyncAll, ProjectCache load/save/discover/check, and automatic cache updates on push/sync.
• Doctor project-issue tests updated to use the cache model.
• make check runs fmt, vet, lint, unit, and integration suites; all green.