feat(graphify): commit-based staleness from built_at_commit (#3170) (#3171)

* test(graphify): TDD-red design contract for #3170 commit-staleness signal

Captures the proposed extension to graphifyStatus() as 8 failing
assertions across 3 groups (git-aware, non-git, back-compat). Suite is
describe.skip()'d so npm test stays green on the branch — removing
.skip is the green-light moment when the enhancement is approved and
implementation lands.

Verified against safishamsi/graphify v0.7.0 release notes: the field
on graph.json is built_at_commit (full git HEAD), not commit_hash as
originally guessed in #3170. Tests assert against the verified name.

Design highlights captured in the file's docstring:
- Tri-state commit_stale (true/false/null) — null means "we don't
  know" (pre-v0.7 graph or no git), distinct from false ("known fresh")
- Argument-injection fence /^[0-9a-f]{4,40}$/i validates built_at_commit
  before it reaches `git` as an argv element
- Existing graphifyStatus() fields (node_count, edge_count, stale,
  age_hours, etc.) are unchanged — back-compat fenced

Per the issue's enhancement template: no PR will be opened until the
issue is labeled `approved-enhancement`.

Refs #3170
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* feat(graphify): surface commit-based staleness from graphify v0.7+ built_at_commit

Closes #3170

graphify v0.7+ embeds built_at_commit (full git HEAD) into graph.json at
write time. GSD's existing graphifyStatus() ignored it; staleness was
mtime-only, which is a poor proxy for "does this graph reflect the
current code." A CI-built graph rebuilt minutes ago against an old
checkout reads as FRESH on mtime but is materially stale.

graphifyStatus() now returns four additional fields on the success path:

  built_at_commit   short hash from graph.built_at_commit, or null
  current_commit    short hash of git HEAD, or null when no git
  commits_behind    git rev-list --count <built>..HEAD, or null
  commit_stale      true | false | null

Tri-state on commit_stale is load-bearing. null means "we don't know"
(pre-v0.7 graph, non-git cwd, unreachable commit) — semantically
distinct from false ("known fresh"). Agents reading null should fall
back to mtime; reading false can confidently skip a rebuild.

Security: built_at_commit is on-disk and user-influenceable. Without
validation, a hostile value (e.g. "--upload-pack=evil") would reach git
as an argv element and be interpreted as an option. The
/^[0-9a-f]{4,40}$/i fence rejects anything else as absent. spawnSync's
array args (no shell) is defense in depth, not the boundary.

Skill (commands/gsd/graphify.md) Step 2b renders one conditional line:

  Source commit: abc1234 (5 commits behind HEAD)
  Source commit: abc1234 (current)
  Source commit: abc1234 (freshness unknown)

Pre-v0.7 graphs omit the line entirely — no confusing "Source commit:
unknown" rendered.

Also documents `graphify hook install` in docs/CONFIGURATION.md for
multi-dev teams who would otherwise hit graph.json merge conflicts on
parallel rebuilds (sub-enhancement 2 from #3170).

TDD red→green: tests/enh-3170-graphify-commit-staleness.test.cjs
(8 assertions across git-aware, non-git, back-compat) was committed
describe.skip()'d in c567f23d when the issue was filed; this commit
removes .skip and lands the implementation that makes them green.
Full suite 7503/7503 passes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

commands/gsd/graphify.md

@@ -100,6 +100,16 @@ node $HOME/.claude/get-shit-done/bin/gsd-tools.cjs graphify status
 Parse the JSON output and display:
 - If `exists: false`, display the message field
 - Otherwise show last build time, node/edge/hyperedge counts, and STALE or FRESH indicator
+- If `built_at_commit` is non-null, also display a `Source commit:` line:
+  - `commit_stale === false` (rebuilt at HEAD): `Source commit: <built_at_commit> (current)`
+  - `commit_stale === true` (graph behind HEAD): `Source commit: <built_at_commit> (<commits_behind> commits behind HEAD)`
+  - `commit_stale === null` (unreachable commit / no git): `Source commit: <built_at_commit> (freshness unknown)`
+- If `built_at_commit` is null (pre-graphify-v0.7 graph), omit the source-commit line entirely — do not render "Source commit: unknown"
+
+The mtime-based STALE/FRESH flag and the commit-based `commit_stale` measure
+different things and can disagree (e.g., a CI-built graph rebuilt minutes ago
+against an old checkout reads as FRESH on mtime but `commit_stale: true`).
+Surface both so the agent can choose.
 
 **STOP** after displaying status. Do not spawn an agent.