Merge upstream/main 13aa882c22 into coder-only (stage 3)

[henrychu04]

Stage 3 of the upstream catch-up: merges upstream/main 13aa882 (38 commits through pingdotgg#20948) into coder-only. Stages 1 and 2 were #201 and #202.

Upstream brought

• Re-render performance work: provider status, reconnect, router, settings and streaming pushes re-render only the consumers whose values changed.
• ChatView overlays moved to ChatOverlayHosts.
• A project action's error now belongs to the request that opened it (fix(web): a project action's error belongs to the request that opened it pingdotgg/t3code#20891).
• Every React test runs in the headless-Chromium browser project.
• Migration 063 drops the v1 projections nothing reads after the v2 upgrade (fix(server): drop V1 projections nothing reads after the V2 upgrade pingdotgg/t3code#20947).

Fork decisions

• V1 screenshots cut (owner decision). Migration 063 is taken verbatim. The fork's LegacyScreenshotArtifacts lookup, workspace.listLegacyScreenshotArtifacts and its timeline row are removed, so imported v1 threads no longer show their saved screenshots, as on upstream.
  • Code that only served that row is also removed: ScreenshotArtifactsRow, CapturedMarkdownImage, ArtifactNavigation, ScreenshotArtifactPreview, and the "artifact" source of workspace.readScreenshotArtifact. That RPC now reads submitted attachments only.
  • Settings → Storage retention can still delete the old copies in $HOME/.t3-coder/artifacts.
• DatabaseMaintenance.ts removed. It is upstream's t3 doctor backend, which feat(server): t3 doctor can delete the database from before 0.1.0 pingdotgg/t3code#20948 extends, and the fork has no t3 doctor.
• RightPanelTabs keeps the fork's version; upstream's per-tab memo (perf(web): right panel tabs no longer all re-render on unrelated chat updates pingdotgg/t3code#20853) is not carried.
• Cut surfaces upstream touched: device setup, load balancing, auth grants, update states, preview-URL action fields, webhook triggers and ACP URL auth.
• Tests: browser tests for cut surfaces are dropped or trimmed. Fork unit tests that upstream deleted stay, under jsdom.

Verification

• Tests:
  • Node: gateway, helper and coder-cli tests pass (119).
  • Packages: effect-codex, contracts, providers, source-control, client-runtime, shared:coder and knip-schemas pass.
  • test:web:coder: 393 files / 6201 tests pass, browser project included.
  • test:server: 270/271 files pass. The one failure is the known serverSettings symlink-watcher flake, which differs from run to run; the file passes alone (56/56).
• Static checks: every typecheck in AGENTS.md, knip:check, pnpm build, and vp fmt --check on the changed files all pass.
• Sync checklist (section 6):
  • Coder: marker counts: the only drop is MessagesTimeline 7 → 6, the cut legacy row.
  • Upstream-added lines missing from the result are all in cut surfaces, the RightPanelTabs memo, or ChatMarkdown fields the fork lacks.
  • Fork-lost lines were reviewed.
• Not done: no live Coder smoke run.

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