feat: cherry-pick checklist categories and --integration doc rename

[maxfindel]

Summary

Two of the five upstream cherry-picks from fellowship-dev/pylot#289. Each adapted to fellowship-dev's terse-templates philosophy — not copy-pasted from upstream.

• Item 2 — checklist philosophy: added the six quality dimensions (Completeness, Clarity, Consistency, Measurability, Coverage, Edge Cases) as explicit categories in templates/commands/checklist.md. The "unit tests for English" framing was already in the file from a prior sync; this just makes the category list complete (Measurability was missing) and tightens the spec-section reference convention. Skipped the ~300 lines of upstream examples, per the issue's terse-templates rule.
• Item 5 — --integration flag: replaced deprecated --ai/--ai-skills with --integration / --integration-options="--skills" in user-facing docs (README.md, CONTRIBUTING.md). Mirrors upstream PR docs: replace deprecated --ai flag with --integration in all documentation github/spec-kit#2359, which is also docs-only — the CLI keeps --ai as a deprecated alias for backward compatibility.

Already done — no change needed

• Item 3 (feature auto-directory naming): already implemented end-to-end in fellowship-dev. templates/commands/specify.md already reads .specify/init-options.json for branch_numbering and supports both sequential (NNN) and timestamp (YYYYMMDD-HHMMSS). The CLI's --branch-numbering flag persists the choice during specify init. Likely landed during the v0.8.1 sync (sync: upstream spec-kit v0.8.1 #2/terse: rewrite v0.8.1 command templates — 1998 → 642 lines #3). Verified by reading both the template logic and src/specify_cli/__init__.py. No template / CLI change needed for this PR.

Deferred

• Item 1 (extension hooks across all 9 command templates): touches every command template; needs a separate review against terse philosophy.
• Item 4 (feature metadata persistence / .specify/feature.json): requires scripts/bash/common.sh changes and a workflow design call.

Note on the issue body

Issue 289 says Item 5 lives in scripts/bash/check-prerequisites.sh. That script does not contain --ai. Upstream PR github#2359 is documentation-only — README, installation docs, CONTRIBUTING. Updated those instead.

Test plan

• bash -n scripts/bash/check-prerequisites.sh clean (script unchanged).
• grep -n '\-\-ai\b' README.md CONTRIBUTING.md returns no hits.
• Existing tests/CI pass.
• Spot-check rendered README sections for the integration examples.

Refs fellowship-dev/pylot#289 — does not close (items 1, 4 still pending; item 3 already implemented).