fix(agenda): yield while preparing agenda entries

[dtvillafana]

Summary

Keep daily/weekly agenda preparation responsive when scanning many large Org files. All configured files and eligible headlines are processed to completion; the work budget controls when to yield, not which entries are included.

This follows the existing notification scanner’s Promise.async, Promise.yield, and OrgFile:parse_async patterns. It is cooperative scheduling, not background-thread processing. Buffer-backed parsing, the parser timeout fallback, and final rendering can still run synchronously.

Related Issues

None linked.

Changes

• Prepare agenda data in an async coroutine, yielding after approximately 5 ms of scanning or date-matching work.
• Use the existing incremental file parser before enumerating headlines, and skip archive files as before.
• Reuse prepared results when rendering, including refresh and date navigation, while preserving agenda cursor/window restoration.
• Avoid synchronous waits when loading custom agenda files, and return an existing search view when preparing it again.
• Add Lua type annotations and six regression tests covering event-loop yielding, incremental parsing and archive exclusion, prepared-result reuse, 6,000 lines across two files, custom-file loading, and refresh/navigation.

The branch contains a single agenda-responsiveness commit on upstream master, without earlier fork-specific changes. No configuration options or user-facing commands change, so no user documentation update is needed.

Checklist

I confirm that I have:

• Followed the Conventional Commits specification.
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary. Added internal annotations; no user documentation changes are necessary.
• Thoroughly tested my changes. Repository-wide stylua --check lua/ tests/ and git diff --check pass.
• Added tests and verified existing tests pass with make test.
• Checked for breaking changes and documented them, if any. None intended.

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