feat(test): bash syntax validation for all SKILL.md code blocks (#1667)

[NikhileshNanduri]

Summary

Closes #1667.

• Adds test/skill-bash-syntax.test.ts: gate-tier free test that runs bash -n on every ```bash ``` block in all 53 generated SKILL.md files (1327+ blocks total), catching syntax errors before they reach users
• Fixes the first bug the test caught: codex/SKILL.md had a missing closing ``` fence on the resumed-session bash block, causing prose with backtick-quoted identifiers to be parsed as shell — bash -n reported "unexpected EOF while looking for matching `"
• Angle-bracket placeholders like <branch-name> are preprocessed to PLACEHOLDER before validation to avoid false positives on intentional template tokens

Why it matters

bun test previously never validated bash syntax in skill files. A typo like 2/dev/null instead of 2>/dev/null (the exact corruption in PR #1654) would pass all CI checks and silently break at runtime. Now it's a gate failure in ~4 seconds with zero API cost.

Test plan

• bun test test/skill-bash-syntax.test.ts — 54 tests pass (1 discovery + 53 skills)
• bun test test/skill-validation.test.ts test/gen-skill-docs.test.ts test/skill-bash-syntax.test.ts — 772 tests pass
• Full free suite passes

[jbetala7]

Thanks for tackling #1667. I think this PR currently collides with #1697 and is hard to land as-is.

Live map evidence:

• Validate skill bash snippets in tests #1697 was opened earlier, also fixes feat: validate bash syntax in SKILL.md code blocks during bun test (prevent silent runtime breakage) #1667, and adds a bash -n validation path for skill bash snippets.
• This PR includes the same user-visible validation goal, but the diff also carries unrelated release/feature work: VERSION, CHANGELOG.md, package.json, AGENTS.md, docs/skills.md, plan-pm-review/SKILL.md(.tmpl), and /autoplan wiring.
• GitHub currently marks this branch DIRTY, so landing it would require separating the feat: validate bash syntax in SKILL.md code blocks during bun test (prevent silent runtime breakage) #1667 fix from older release-wave changes.

Recommended path: make #1697 the canonical PR for the bash-snippet validation gate, or rebase/narrow this PR down to only the #1667 pieces:

1. the codex/SKILL.md.tmpl fence fix,
2. the bash syntax validation test or equivalent,
3. no VERSION/CHANGELOG/plan-pm-review/autoplan changes.

The important coverage to preserve is a regression that catches both the missing Codex fence and the 2>/dev/null -> 2/dev/null corruption pattern from #1654.