chore(pm): raise the os-dev.md line ceiling 399→405 per the 2026-08-23 maintainer ruling

The merge group for this PR went red on check-skill-line-ratchet: os-dev.md
composes to 401 lines against main (which #11112 took to 395), and to 405 once
the queued #11137 (395→399) lands ahead of it — each PR green alone, over the
ceiling only in composition. Maintainer ruling, 2026-08-23, on this PR
(option B of the three put to them, verbatim): 「B：提天花板 399→405」 —
keep the +6-line cross-repo caveat and raise the ceiling, quoted in the PR
body as the ratchet requires. Sized to exactly 405 so headroom returns to
zero once both queued PRs land.

Gate + 19-case self-test green at this commit.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MsbKEG4LtERSLaDrbehM3e

Author: claude

scripts/pm/check-skill-line-ratchet.mjs

@@ -112,7 +112,10 @@ export const CEILINGS = new Map([
   // platform capabilities implemented upstream, 展现平台能力, 不扩散需求, runs on
   // community edition). Set at landed line count (headroom 0, same convention).
   ['.claude/skills/pm-dispatch/references/lanes/hotcrm.md', 45],
-  ['.claude/agents/os-dev.md', 399],
+  // 399 → 405 (#11126): maintainer-ruled (2026-08-23, option B, quoted in that
+  // PR) — the +6-line cross-repo dispatch-gates caveat, sized so the queued
+  // #11137 (395→399 on main) and this PR's +6 compose to exactly 405.
+  ['.claude/agents/os-dev.md', 405],
   // #9473: the other four `.claude/skills/` are read in full by the sessions
   // that use them too — the erosion mechanism the ratchet exists to stop
   // isn't specific to the pm-dispatch surface. Set at current counts on