docs/now/README.md: describe the shape gate, the weaker local hooks, and the Archive-Repair rule #5961

docs/now/README.md is the single home for the docs/now/ entry layout, and CONTRIBUTING.md is being rewritten to point at it (#5950, PR #5952). Four of its statements no longer match the code (line numbers against origin/master 93868bf):

1. The shape gate is never mentioned. .github/workflows/check-now-freshness.yml:36 (job check) runs tools/check_now_entry_shape.py, whose docstring (lines 1-45) defines the entry shape. Its filename pattern, tools/check_now_entry_shape.py:56 ([a-z0-9][a-z0-9-]*), is stricter than the sync gate's, scripts/ci/now-sync-gate-diff.sh:56 ([A-Za-z0-9._-]+), so a name one gate accepts can fail the other.
2. README lines 68-70 say .githooks/pre-commit, scripts/pre-commit, t27c check-now and tri hooks now-gate enforce "the same three conditions" as the CI sync gate. They only check freshness of the whole docs/now/ directory (bootstrap/src/suite.rs:3158-3175, cli/tri/src/hooks.rs:298, scripts/pre-commit:14-50), and cli/tri/src/nownote.rs:265-273 and :294-298 say so: any in-window entry already on master satisfies them.
3. README line 82 says docs/NOW.md "is no longer read by any gate", but scripts/ci/now-sync-gate-diff.sh:126-179 refuses a range that edits it unless a commit carries an Archive-Repair: trailer.
4. The README does not say that tri now add needs the Rust tri binary (cargo build --release -p tri; scripts/tri:83, :528) or that a hand-written entry works, and it does not mention tri now check (cli/tri/src/nownote.rs:260) or tri hooks pre-push (cli/tri/src/hooks.rs:45-56).

Fix: docs only. Point at the checker's docstring rather than copy the shape, and change no gate, hook, workflow or tool.