feat(substrate): implement M4 goal-anchoring — the one missing paper capability

[CodeWithJuber]

What & why

The white paper defines 11 capabilities (5 faculties + 6 mechanisms). Auditing forge against them, ten already ship a real mechanism — but M4 goal-anchoring was documented (docs → "M4 goal-anchoring → substrate") yet never implemented. substrateCheck had zero drift logic. This PR builds it, so all 11 now map to something real. Done with a ponytail discipline — smallest correct thing, reuse what's here, keep the rest untouched.

New: src/anchor.js → goalDrift(root, goal) — a deterministic goal-drift check (the paper's "re-read the goal against in-progress work"):

• Compares the stated objective against the files you've actually changed (git diff HEAD + untracked), splitting them on-goal vs off-goal.
• Reuses referencedEntities (goal keywords/symbols/files) and the atlas — a symbol-named goal (change verifyToken) anchors to the file its symbol lives in (src/auth.js), which name-matching alone would miss.
• Filters out forge's own generated footprint (.forge/, emitted tool configs, AGENTS/CLAUDE.md) so it flags your work, not forge's.
• Coarse and advisory by design (path/keyword match, not semantic); the ceiling is named in a ponytail: comment.

Surfaced two ways:

• forge anchor "<goal>" [--json] — standalone mid-session "am I still on track?" check.
• Folded into forge substrate (result.goalAnchor): quiet on a clean tree, speaks only once a diff has drifted — in both the full render and the ambient Claude-Code advisory.

Live example (two files changed, goal names one):

$ forge anchor "harden verifyToken in src/auth.js"
  changed: 2 file(s) · on-goal 1 · off-goal 1
  off-goal (unrelated to the stated goal — intended, or drift?):
    - src/report.js

Docs made honest: the 11-point mapping now points M4 at anchor; README command list, docs/GUIDE.md (worked example + everyday-workflow step), the substrate README checks table, and CHANGELOG.md all updated.

Checklist

• npm test passes — 151/151 (4 new test/anchor.test.js: off-goal flagged, on-goal quiet, no-diff quiet, pure-drift flagged)
• npm run check passes (Biome lint + format; new files formatted)
• New public function has a test (goalDrift)
• Conventional commit message (feat:)
• CHANGELOG.md updated under ## [Unreleased]
• No new runtime dependency (zero-dep: node:child_process + existing modules)
• Substrate/docs updated (this changes forge substrate output + adds forge anchor)

Risk & rollback

• Risk level: low — additive; the new field is quiet unless there's a git diff that drifts, and it's advisory (never blocks, never fails the process). No existing behavior changed.
• Rollback plan: revert this commit; forge anchor and result.goalAnchor disappear, nothing else depends on them.

Extra checks (tick if applicable)

• npm run typecheck passes (JSDoc-typed the injectable changed seam)
• Input validated at boundaries; errors handled (git calls are try/caught → empty)
• Logs contain no secrets/PII

🤖 Generated with Claude Code

https://claude.ai/code/session_01Eh3SNx15EeVbTmU4nH36XC

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