refactor(desktop): quiet the app shell chrome - #238

Outcome

Nine chrome marks from the user's review of the running build:

• one 4px gap across the titlebar toolbar (was 16px between clusters, 8px inside the action group,
  and a 17px trailing gap before the layout control);
• the project mark paints no tile and no ring, and the session header drops its leading project mark
  entirely so the breadcrumb starts with the project name;
• the rail's checkout and pull-request badges lose their pill fill and padding so their text starts on
  the row title's edge, and the pull-request state now travels by colour, icon and tooltip instead of
  a visible "Merged" / "CI failed" word;
• the selected rail row keeps its selected fill without the leading bar;
• the compact composer renders the session chips in the single control row with + / voice / send,
  and the checkout bar's labels move to the quiet grey.

Change record

• Change: docs/sdlc/changes/2026-09-16-quiet-shell-chrome/intent.md

The linked change directory owns scope, authorization, acceptance, evidence, and residual risk.

Review and release

• Acceptance evidence covers the current diff, including rendered UI where affected: the toolbar
  gaps measured 4/4 against 16/8/17 before, the checkout chip computed to the muted foreground
  grey, the header mark is absent from the rendered shell, and the record's AC-1..AC-9 map to
  current PASS evidence (rail rows and branch chip are structural plus rendered tests, since the
  Core-less renderer has no session data).
• Required checks passed on this base: bun run lint, bunx tsc --noEmit (inside
  build:renderer), bun test (957 pass / 3 skip / 0 fail), bun run build:renderer,
  bun script/verify/sdlc.ts --worktree, bun script/verify/docs.ts, git diff --check.
• Merge authorization: pending (this PR was requested; merge is not authorized)
• Release authorization: not requested