Show where each change's issue, branch, and worktree live

[androidand]

Summary

• Adds specsync topology: a read-only join of every OpenSpec change with its tracker issue, git branch, and worktree path across coordinated stores — so an agent can check what's already in flight before starting, instead of ad-hoc scanning git/gh state itself.
• Every source is independently optional: no openspec binary, no gh auth, an uncloned sibling store, or a store with zero worktrees each narrow the affected rows and append a reason to status[] rather than failing the command. Exits non-zero only when nothing could be resolved at all.
• linked resolves from links.md against a local slug index built across every loaded change. epic resolves an otherwise-unmatched related link by confirming the referenced issue's type:epic label via one bounded gh call — specsync epic only writes a remote backlink today (never a local one), so this is the only reliable signal available.
• Adds the "check topology before starting" step to the embedded SKILL.md, propagated byte-identically to all 4 copies (verified by TestSkillDrift), plus a site/features.json entry and a README mention.
• Also fixes an unrelated, pre-existing test bug: TestPull_CannotResolveFromBranch relied on whatever branch the suite happened to be run from instead of overriding currentBranchFn like every other branch-resolution test — it failed whenever run from a feat/<n>-<slug>-named branch (i.e. this one).

Closes #168

Test plan

• go build ./..., go vet ./..., gofmt -l clean on every touched file
• go test ./... — 806 passed, 0 failed
• New unit tests for BuildTopology: fully resolved single store, no-worktree row still returned with exit 0, gh-unauthenticated degradation + status note, nothing-resolved is an empty result not an error, read-only (no cache/state written), -change scoping, archived exclusion + -all, multi-repo store join, linked resolution, epic resolution via label
• New unit tests for listWorktrees/matchWorktree: regular/detached/bare porcelain parsing; matched by branch, matched by basename fallback, unmatched
• New unit tests for printTopologyTable in cmd/specsync
• Dogfooded against this repo: specsync topology correctly listed ~29 changes including this very change, with its real branch (feat/168-agent-topology-command) and worktree path (the worktree it was written in); specsync topology -json -change agent-topology-command with a local ref cache resolved live issue state via a real gh call; re-run with gh removed from PATH degraded exactly as documented (state omitted, status note present, everything else still resolved)
• Timed: ~1.5s wall-clock for this repo's ~29 changes, one store, no gh calls — acceptable for an agent-startup check
• specsync changelog -resolve-refs renders this commit under "Added" via the proposal's title

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