feat(storage): reclaim free runtime SQLite pages in Host maintenance

[garvit-arora]

Summary

Add a bounded Host maintenance lane that returns free pages from new runtime.sqlite databases initialized with auto_vacuum = INCREMENTAL by #6044. Each tick reads auto_vacuum and freelist_count in a read transaction, skips older NONE files without a write lock, runs at most 64 pages of incremental_vacuum through the process-local operational database owner, and follows a productive commit with a PASSIVE checkpoint. The lane uses the existing 100 ms active and 60 s idle cadence and drains with Host maintenance.

Existing NONE databases remain report-only. This PR does not convert them, add another SQLite owner connection, or change protocol, UI, or compatibility epoch.

Refs #6000

Verification

• Core, storage, MCP, runtime, and Runtime Host builds passed on the initial rebased head; storage and Runtime Host builds passed again after adding the tests.
• Focused storage and Host maintenance tests: 10 passed. They cover the 64-page limit, draining an incremental database, preserving an existing NONE database and its freelist, invalid limits, and Host lane scheduling.
• Biome lint and format passed on the changed test files; git diff --check passed.
• Full PR CI is pending on the latest head.

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex helped implement and review the storage and maintenance lane and its focused tests.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — new incremental runtime databases can reclaim free pages in Host maintenance.
• No