feat(task): let agent frontmatter opt subagents into their own todo list

[F0Rextasy]

What

I reviewed the full diff; subagents could never keep the todo tool because two layers stripped it unconditionally, so agent frontmatter todo: true plus a tools grant containing todo now keeps it while the default stays parent-owned.

Why

Fixes #12575

Testing

• bun test packages/coding-agent/test/tools/index.test.ts -t todo (2 pass, 1 new: explicit grant keeps todo, ungranted stays stripped)
• bun --cwd=packages/coding-agent run check:types (pass)
• oxlint + oxfmt on all 6 changed files (pass)

---

• bun check passes
• Tested locally
• CHANGELOG updated with the required attribution (if user-facing; internal issue fixes use issue links, external contributions add the PR link and contributor credit after creation)

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