docs(tui): record grouped timeline plan and baseline

[jlongster]

Summary

• Record the sequential grouped-timeline implementation plan and small PR boundaries.
• Document the clean v2 production baseline, current grouping behavior, and verification evidence.
• Establish acceptance criteria for the next, separate shared-renderer extraction PR.

Documentation only; no runtime or test code changes. Diagnostic scripts and screenshots remain local, with paths recorded in the document.

Verification

• bun typecheck from packages/tui: passed
• bun run test from packages/tui: 1352 passed, 4 skipped, 0 failed
• OpenCode Drive with --dev: inspected busy exploration and completed thought/exploration at 112x34 and 80x24
• git diff --cached --check: passed

Baseline revision: 8f4d7066473ea07d26c5dfc35e46cd9a94e3e292. Full visual parity checks accompany subsequent implementation PRs.

[jlongster]

Closing: this plan and baseline log will remain local. The first code PR will contain the behavior-preserving session-renderer extraction.