chore(sync): merge upstream/main into fork

[adampeterhiggins]

Sync the fork with all 28 upstream commits through 6524a8d437. At the start of the sync, the fork was 330 commits ahead and 28 behind upstream. This branch is based on fork main through 532533043e, retaining its filter menus, linked-repository PR references, model-parameter change markers, persistent sidebar searches, and lockfile-aware update builds.

Resolved the overlapping changes while keeping fork behavior:

• Claude subagents retain final usage and output-file links, including upstream's new idle completion path. Added coverage for completed, failed, and stopped idle agents, alongside upstream's native-outcome and resume tests.
• Stop still supports keeping delegated tasks running, while targeted native-agent stops use upstream's confirmed outcomes.
• Database startup repairs the fork's legacy migration 55 before upstream's new migration-history guard. Both migration suites pass.
• Git actions use upstream's callbacks bound to the selected thread, retaining the fork's browser-opening action and preferred PR-link behavior.
• Composer context retains fork tab capture and reference attachments alongside upstream's pending-answer attachment targets.
• Web/mobile tool presentation keeps fork integration marks, rich previews, file reads, and syntax highlighting alongside upstream's REPL titles, highlighting, and computer-use fixes.
• Mobile PR lists keep the fork's filter facets and upstream's connected-environment preference.

Audited the automatically merged intersections and docs/fork-differences.md; none of the documented differences was adopted or removed by these upstream commits. Devin, Custom ACP, Cursor defaults/steering, integration context, chat tabs, agent panels, update pausing, and other fork additions remain in the merged tree.

Validation:

• Focused regression runs covering more than 1,800 test executions: Claude/Codex adapters, Stop/effect worker, database migrations/event replay, Git actions/PR review guides, MCP access/delegation, Devin/Custom ACP, tabs, usage, agent fleets, filter facets, composer PR lookup, remote fallback/reconnect, attachments, and desktop updates. All selected suites pass; the route-search test was run with the web package's Vite config.
• Contracts, client-runtime, provider-core, mobile, and desktop package typechecks pass. After updating the base, another 428 focused tests pass for model-change markers and traits, sidebar behavior, mobile activity, PR filtering, and delegated-task launch; client-runtime/mobile typechecks and the web check with the ES2024 library also pass.
• Web typecheck passes with --lib ES2024,DOM,DOM.Iterable. Its standard check has a pre-existing ES2023-library error on Promise.withResolvers in terminal/ghostty/surface.test.ts.
• Server typecheck has pre-existing middleware inference errors in unchanged mcp/AgentAccessMcpServer.ts; these are disclosed rather than counted as a clean check.
• Server production bundle and final web production build pass.
• Targeted lint/format checks have no errors; git diff --check passes. No browser or native-device verification was performed.

Merge with Create a merge commit. It keeps upstream history and the merge base for the next sync.

Model: GPT-6.1-sol. Harness: Codex in T3 Code.