feat(web): add compact sidebar rail and compact thread row density (#11525, #9417, #11644, #11652, #11595)

[rynfar]

Summary

Ports upstream T3 Code commits for compact sidebar rail and compact thread row density (Cycle #669 Group B):

• #11525: Add optional compact sidebar rail (compactSidebarEnabled, default false)
• #9417: Add compact thread list mode (sidebarCompactThreadRows, default false)
• #11644: Refine compact thread row badges
• #11652: Show the linked pull request in the compact sidebar rail
• #11595: Keep sparse sidebar shelves at the bottom

Pylon Divergence & Quality Guarantees

• Both settings default to false, preserving existing Pylon sidebar appearance and behavior until explicitly opted in.
• Preserved Pylon's pull request presentation model (resolveThreadPullRequestBadgePresentation) and right-panel stack integration.
• Preserved Pylon branding ("Pylon", not "T3 Code").
• Preserved Pylon settings (showProviderUsageInContextPopover).
• No revival of pair mode / delegation UI removed in refactor: remove Pylon pair mode and thread delegation #673.
• Adversarial audit addressed:
  • Ensured plan-ready thread status renders an accessible status dot and label in compact row density mode.
  • Omitted phantom settled markers from drag layout projection in compact rail mode.
  • Added explicit test fixture for compactSidebarEnabled.

Closes #669.

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Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.9 KiB	14.0 KiB	+26 B (+0.2%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.2 KiB	7.2 KiB	+12 B (+0.2%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.7 KiB	6.7 KiB	+14 B (+0.2%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	58.0 KiB	58.0 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	13.9 KiB	14.0 KiB	+22 B (+0.2%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.2 KiB	7.2 KiB	+8 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.7 KiB	6.7 KiB	+14 B (+0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.9 KiB	58.9 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: efff40d · PR result: 4e0f463 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 115.7 KiB
• Claude decoded thread snapshot: 116.4 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.