feat(settings): expose subagent execution timeout

[changer-changer]

Subagent runs already honor agent.subagents.timeoutMs, but users must edit YAML to change it. This adds a maximum run time field, in seconds, to the existing Agent Model advanced settings, with English and Chinese help text. Clearing the field restores the existing one-hour default and preserves other subagent settings.

The native number input accepts optional bounds so invalid or overflowing durations cannot be committed. Fractional seconds round to milliseconds; the backend timeout mechanism and default behavior are unchanged. This change is independent of #576.

Validation:

• Agent Model tests: 23 passed, including 8 duration cases covering conversion, reset, invalid values, cancellation, and preservation of sibling settings.
• Web regression with the repository's CI exclusions and --maxWorkers 2: 191 files / 1,533 tests passed.
• UI production build passed.
• Browser interaction with the actual settings component confirmed 90.125 seconds becomes 90125 milliseconds, zero cannot be saved, and clearing removes only the override. Production config normalization preserves the value.

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Merge sequencing (added 2026-09-14)

This PR is one of four coordinated subagent-lifecycle PRs (#579, #576, #580, #581). The features are semantically independent but overlap textually in the agent core, so an explicit merge order is proposed in #583.

Proposed order: #579 → #576 → #580 → #581 (this PR: 1 (first)).

This branch's known conflicts: clean against current main and against #580/#581; a single shared UI file with #576 (modelPool/types/index.ts).

After each sibling PR merges, I will rebase the remaining branches onto main and re-run the targeted test suites before updating the PRs.