chore: scrub pre-GitHub dev-process references from docs and tests

[adworzynski]

Follow-up to deleting the archive/pre-github tag: the pre-GitHub development apparatus (the PRD,
numbered specs, the D-decision log, phase-gate evidence) no longer exists in the repo, so every
reference to it is now a dead pointer. This scrubs them so nothing in the public repo points at
deleted content.

Public-facing (dead instructions removed):

• README.md, docs/README.md, docs/decisions/README.md no longer tell readers to find history
  "at the git tag archive/pre-github"; stale .gitignore comment reworded.

Test docstrings + proposals (de-archaeology):

• Dropped Phase-N gate / gate evidence framing and the 23a/b/c phase signatures.
• Dropped dead citations to the deleted numbered specs (spec NN §X), the D-decision log
  (DNN), and Q-questions (QNN).
• Remapped the determinism NFR (NFR-1) to P1, the principle that still documents it.
• Stale gate19 → test_skill_parity; per-gate e2e suites → per-feature.

Kept: legitimate uses of "gate" (P5 Proof-Gated Growth, CI gates, the mandatory ea pack validate
gate) and all live principle (P1–P8) references.

Docstrings/comments/docs only — no code changed, 358 tests green, mkdocs build clean.