Make the resume-state doc a required, checked Core artifact

[PBNZ]

Retrospective finding — refs #10.

Context

The single most common breakdown in a nine-repo retrospective: resume-state documentation decays
the moment the build phase ends — or never exists. Observed (anonymised):

• Three repos have no resume-state doc at all; their own START-HERE maps don't list one, so
  the gap is invisible to a fresh session.
• One repo's checkpoint and roadmap froze on the day the repo went public — while several more
  releases shipped over the following two weeks. The "next step" it records was completed long
  ago.
• One living-docs repo's state facts went stale within 48 hours of adoption — and the response
  was to loosen the CI staleness gate rather than refresh the doc.
• The two repos that stayed current prove the discipline works: one keeps a model checkpoint with
  an explicit "in progress: nothing / exact next command" section; another substitutes a
  README-status contract, documents the substitution, and honours it.

The standard asks for a current resume-state doc but the Core tier neither requires the file
nor verifies it — so compliance depends entirely on session habits, which fail exactly when a
project pauses (the moment the doc matters most).

Proposal

1. Core tier requires a resume-state artifact — checkpoint file, living-docs state file, or a
   declared substitute — with a mandatory row in the START-HERE map.
2. Header contract: the artifact carries a "last updated" date and an explicit next step
   (even "paused — nothing pending").
3. Checklist tripwire: the pre-commit checklist gets a hard line — "resume-state updated, or
   this commit doesn't change state (say which)".
4. Optional CI nudge: warn (not fail) when the resume-state file is untouched across N
   commits or M days of activity.
5. Write the meta-rule down where the living-docs add-on defines its checks: when a
   freshness gate goes red, fix the doc — never widen the gate. The gate-loosening observed was
   rational in the moment precisely because no rule said otherwise.

Tasks

• Update the Core tier file list + START-HERE guidance.
• Add the checklist line; add the CI nudge as an opt-in snippet.
• Add the "fix the doc, not the gate" rule to the living-docs reference.

Acceptance criteria

• A scaffolded repo cannot pass its own checklist without a resume-state artifact.
• The staleness nudge exists as a copy-paste snippet; repos can adopt it without bespoke work.

— 🤖 Claude, on behalf of @PBNZ

[PBNZ]

Triage: accepted (adapted: the CI staleness nudge ships as a documented opt-in snippet, not a scaffold default — warn-level noise shouldn't be mandatory at Core).

Landed on PR #18 (commit fd47702): Core now requires a resume-state artifact — docs/CHECKPOINT.md by default (stamped by /new-repo with last-updated + explicit next step), or a declared substitute with a mandatory START-HERE row. The pre-commit checklist gains the "resume-state updated, or this commit doesn't change state — say which" tripwire; living-docs.md gains the "when a freshness gate goes red, fix the doc — never widen the gate" rule; repokit-check.ps1 (#16) enforces the row mechanically. repo-kit dogfoods its own checkpoint.

— 🤖 Claude, on behalf of @PBNZ

[PBNZ]

Verified on main (f7ab31c): docs/CHECKPOINT.md template + mandatory START-HERE row in the standard, pre-commit tripwire, gate rule in living-docs.md; repokit-check.ps1 enforces the row and passes on this repo (CI green). Closing on @PBNZ's instruction.

— 🤖 Claude, on behalf of @PBNZ