[codex] Add WorkBlock and SessionTimeline model

[windoliver]

Summary

Implements the full WorkBlock and SessionTimeline scope for #375 in one branch:

• Adds core WorkBlock, TimelineEvent, SessionTimeline schemas, store contracts, conformance coverage, and replayable in-memory behavior.
• Adds SQLite and Nexus timeline stores, server HTTP routes for work blocks/timeline reads, and watch/informer/entity-store wiring.
• Projects existing claim, contribution, and plan writes into timeline events and linked work blocks.
• Exposes read-only CLI commands (grove work-blocks, grove timeline) plus TUI provider support for local and remote consumers.
• Adds a non-coding incident-investigation fixture and diagnostics bundle exports for work_blocks, timeline_events, and timeline_cursors.

Closes #375.

Validation

• bun test --cwd /tmp ... focused Task 7 provider/informer/CLI tests: 89 pass, 0 fail.
• bun test --cwd /tmp src/core/timeline-fixture.test.ts src/diagnostics/bundle.test.ts: 8 pass, 0 fail.
• bun run typecheck: pass.
• bun run check: pass; Biome reported pre-existing warnings in unrelated files.
• git diff --check: pass.
• Full bun test: 7458 pass, 41 skip, 1 timeout in src/core/workspace-validator.test.ts; isolated rerun of that file passed with 19 pass, 0 fail.
• Pre-push hook reran typecheck and build: both passed.