Add time-budgeted metadata reconciliation drain

[chubes4]

Summary

• Adds --until-budget=<duration> support for reconcile-metadata --apply so DMC can drain bounded pages until a time budget is nearly exhausted.
• Returns continuation evidence including next offset and next command for safe resume.
• Uses the direct bounded apply path from Support bounded metadata reconciliation apply #345 and does not require shell-managed plan files.

Closes #341.

Stacking

• Stacked on Support bounded metadata reconciliation apply #345 (fix/343-direct-reconcile-apply) because the drain uses direct bounded apply output.

Testing

• php tests/smoke-worktree-metadata-reconcile.php
• php -l on changed PHP files
• homeboy lint --path . --changed-only --errors-only --force-hot

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted and implemented the time-budgeted reconciliation drain and smoke coverage; Chris retains review and merge responsibility.