docs: update project documentation for v1.69.1.0

CLAUDE.md: Token ceiling section documents the context-budget ratchet as
the third guard (test file, fixture, new-skill budgeting, capture command).
CONTRIBUTING.md: Tier 1 guard list gains a Context-budget ratchet bullet;
the Adding-a-new-skill checklist gains the budget-capture step.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -226,6 +226,16 @@ conversation. The failure message carries the re-measure + ratchet protocol.
 tree (always-on vs per-invocation, `--diff`, `--budget`; `--exact` opts into the
 real tokenizer and POSTs file text to api.anthropic.com with an egress receipt).
 
+A third enforced ceiling is the context-budget ratchet:
+`test/context-budget-ratchet.test.ts` (free, runs in `bun run test`) grades the
+two ledgers the other guards don't cover — the always-on FULL-frontmatter
+aggregate and each skill's per-invocation eager tokens (SKILL.md + forced-read
+references) — against `test/fixtures/context-budget.json`. A skill that grows
+past its ceiling fails; a new skill fails until it's consciously budgeted. For
+legitimate growth or a landed reduction, re-run
+`bun test/helpers/capture-context-budget.ts` and commit the refreshed fixture
+in the same commit, so ceilings ratchet down and every win is locked.
+
 **Merge conflicts on SKILL.md files:** NEVER resolve conflicts on generated SKILL.md
 files by accepting either side. Instead: (1) resolve conflicts on the `.tmpl` templates
 and `scripts/gen-skill-docs.ts` (the sources of truth), (2) run `bun run gen:skill-docs`