feat(web): expose each chat message as a heading for screen readers (#11199)

[rynfar]

Part of #669 (Group A: chat message accessibility).

Upstream source

• Upstream commit: 6cdbf76fa4b0509913046abe6105037ded9e52bb
• Upstream PR: feat(web): expose each chat message as a heading for screen readers pingdotgg/t3code#11199

Summary

Exposes each chat message and proposed plan card as a heading for screen readers, allowing users to skim the transcript by author using standard heading navigation:

• Introduces MessageAuthorHeading with h3.sr-only.select-none in MessagesTimeline.tsx.
• Preserves Pylon branding: User announces as "You", Assistant announces as "Pylon".
• Adds headingLevelOffset prop to ChatMarkdown, applying an aria-level offset to nested headings without altering HTML heading tags or visual presentation.
• Adapts ProposedPlanCard title to h3 with headingLevelOffset={3}.
• Compatible with recent pair/delegation removal (refactor: remove Pylon pair mode and thread delegation #673).

Verification

• Focused unit tests: vp test run apps/web/src/components/ChatMarkdown.test.tsx apps/web/src/components/chat/MessagesTimeline.test.tsx (111 passed).
• Typecheck: vp run -F @t3tools/web typecheck (0 errors).
• Scoped checks: vp check (0 errors).
• Adversarial review: Passed and approved with no regressions.

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1 Skipped Deployment

Project	Deployment	Actions	Updated
pylon-marketing	Ignored		Sep 20, 2026 8:15am UTC

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	14.0 KiB	14.0 KiB	+10 B (+0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.2 KiB	7.2 KiB	−14 B (−0.2%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.7 KiB	6.8 KiB	+24 B (+0.3%)	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	14.0 KiB	14.0 KiB	+6 B (+0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.2 KiB	7.2 KiB	−4 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.7 KiB	6.7 KiB	+10 B (+0.1%)	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: 628e1b7 · PR result: ea45ac3 · 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: 115.6 KiB
• Claude decoded thread snapshot: 116.3 KiB

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