Add per-turn timer and todo budget feedback

[0vp]

Summary

Port the standard --timer and TodoWrite step-budget feedback from the old Espri-API eval-max-timeout branch into the current provider-agnostic harness. The source feature began in 23f906de2 (turn timer) and 49fef5b18 (todo allocations); this includes the branch's later schedule-accounting fixes.

• Add --timer <seconds>, --timer off/0, --no-timer, and persisted /timer <seconds> / /timer preferences. CLI overrides do not overwrite saved preferences.
• Re-arm an advisory wall-clock budget each turn. Append remaining-time notices at 50%, 25%, and 10% to the last output of a completed tool round. Background Execute launches force a fresh clock report.
• Add optional timeBudgetSeconds to todo items. Report step duration against the allowance captured at activation, completed-step count, and remaining allocations. Flag material overcommit once per turn with the source branch's max(60 seconds, 10%) tolerance.
• Preserve allocations across status-only updates and session resume. Filter explicitly tagged timer context from resumed display messages while retaining it in provider/model history.

Harness boundaries

• Keep timer state, schedule accounting, outbound decoration, and prompt wording in separate modules.
• Decorate copied outbound results only after ordinary transcript recording. Keep cancellation, retries, tool IDs, and error prefixes intact.
• Put each turn's budget on its user message, not the cached system prefix. The prompt explicitly describes advisory behavior rather than claiming the CLI enforces termination.
• Validate positive whole seconds consistently; malformed /timer input does not clear a saved budget.
• No supervisor model, eval-runner changes, automatic command handback, Execute timeout redesign, or changes to sub-agent execution limits.

Validation

• bun run validate: 1,675 passed, 9 platform-dependent skipped, 0 failed. Includes lint and both typechecks; passed again in the pre-commit hook.
• Compiled with the repository build's bun build --compile command, using a temporary output path to preserve the existing local binary.
• Compiled backboard --help smoke test passed.
• Unit/integration coverage includes thresholds, delayed reports, per-turn reset, todo accounting, preference persistence, wire-only decoration, and resume behavior.
• No live paid-model smoke test was run.