#1174 Committed .swamp-sources.yaml relative paths resolve against worktree cwd, not the main checkout — and missing sources are now skipped silently

Summary

.swamp-sources.yaml relative path: entries are resolved against the process cwd (the worktree's physical location), not the repository's main checkout. Inside a nested git worktree (e.g. .claude/worktrees/<name>), a committed sibling source like ../spor-infra/extensions resolves to a nonexistent path — and as of the current build the missing source is silently skipped (exit 0, no warning at default log level), so type-touching operations quietly run without the sibling's extensions instead of failing loudly.

Steps to reproduce

1. A repo ~/repos/my-repo with a committed .swamp-sources.yaml:

   sources:
     - path: ../sibling-repo/extensions
       only: [models, reports]

   and the sibling checkout at ~/repos/sibling-repo (works fine from the main checkout).
2. Create a nested worktree: git -C ~/repos/my-repo worktree add --detach .claude/worktrees/test HEAD
3. cd ~/repos/my-repo/.claude/worktrees/test && swamp model list

Observed

• swamp attempts to open ~/repos/my-repo/.claude/worktrees/sibling-repo/extensions/.swamp.yaml (verified with strace -e trace=openat: 3× ENOENT) — i.e. ../sibling-repo relative to the worktree directory.
• The command exits 0 with no warning; models still list from the datastore, but the sibling source's model types are not loaded. In an earlier build this failed loudly; the silent skip is worse because it can mask stale-type behavior in scripted/CI use.

Expected

Relative path: entries in a committed .swamp-sources.yaml should resolve against the repository's main checkout — e.g. against git rev-parse --git-common-dir's parent (or git worktree list's first entry) — so every worktree of a repo sees the same sibling sources the main checkout does. A committed file cannot carry per-worktree paths, so cwd-relative resolution makes worktree use impossible without filesystem workarounds. Failing that, an unresolvable declared source should at least warn loudly rather than skip silently.

Workaround in use

A symlink at the worktrees container level (.claude/worktrees/sibling-repo -> ~/repos/sibling-repo) so ../sibling-repo happens to resolve from every worktree in that directory. Works, but it's gitignored local state every contributor must rediscover.

Environment

• swamp 20260716.010800.0-sha.41b2f5f0 (also present in 20260627.002632.0, where it failed loudly instead of silently)
• Linux 6.12, git worktrees nested under the main checkout at .claude/worktrees/<name>
• Note: SWAMP_EXTENSIONS_DIR / --extensions-dir (#381/#388) covers the local extensions/ dir split, but does not affect .swamp-sources.yaml path: resolution, which is what breaks here.