Implement five backlog workspace UX improvements

[raphaeltm]

Summary

Implemented five backlog UX tasks in one PR (workspace-focused quality-of-life fixes):

• Reliable git status badge: added retry-on-initial-load, 30s polling while running, stale indicator, and GitChangesPanel -> header badge synchronization.
• Mobile command palette access: added dedicated mobile search button and compact utility-button sizing to fit the header.
• Markdown file rendering: added rendered/source toggle for .md/.mdx, persisted preference (sam:md-render-mode), and markdown styling with GFM support.
• Close last terminal tab behavior: removed synthetic/unclosable terminal tab logic; closing terminal tabs now behaves like normal tab lifecycle and preserves +-menu creation flow.
• PWA in-app navigation: already implemented on main (standalone-mode path uses in-app navigation), validated and archived backlog task to remove tracking drift.

Also moved the five completed backlog files from tasks/backlog/ to tasks/archive/.

Validation

• pnpm --filter @simple-agent-manager/web lint
• pnpm --filter @simple-agent-manager/web typecheck
• pnpm --filter @simple-agent-manager/web test
• Additional validation run (if applicable)
• Mobile and desktop verification notes added for UI changes

Additional validation:

• pnpm --filter @simple-agent-manager/shared build
• pnpm --filter @simple-agent-manager/ui build
• pnpm --filter @simple-agent-manager/terminal build
• pnpm --filter @simple-agent-manager/acp-client build
• pnpm --filter @simple-agent-manager/web build
• Mobile viewport Playwright harness screenshot saved to .codex/tmp/playwright-screenshots/mobile-ui-qol-harness.png

UI Compliance Checklist (Required for UI changes)

• Mobile-first layout verified
• Accessibility checks completed
• Shared UI components used or exception documented

Exceptions (If any)

• Scope: Playwright verification used a mock harness for authenticated workspace surfaces.
• Rationale: The changed workspace views require auth/runtime state; harness was used per agent guidance for auth-gated UI.
• Expiration: N/A

Agent Preflight (Required)

• Preflight completed before code changes

Classification

• external-api-change
• cross-component-change
• business-logic-change
• public-surface-change
• docs-sync-change
• security-sensitive-change
• ui-change
• infra-change

External References

N/A: This change is internal UI behavior and existing codepaths; no external API/library behavior research was required.

Codebase Impact Analysis

• apps/web/src/pages/Workspace.tsx
• apps/web/src/components/FileViewerPanel.tsx
• apps/web/src/components/WorkspaceTabStrip.tsx
• apps/web/src/components/GitChangesPanel.tsx
• apps/web/src/components/GitChangesButton.tsx
• apps/web/src/components/FileBrowserButton.tsx
• apps/web/src/components/CommandPaletteButton.tsx (new)
• apps/web/tests/unit/... (workspace/file viewer/git/tab strip/new button tests)
• tasks/archive/* (moved 5 completed backlog tasks)

Documentation & Specs

• Updated task tracking docs by moving completed files:
  • tasks/archive/2026-02-17-reliable-git-status-badge.md
  • tasks/archive/2026-02-17-mobile-command-palette-access.md
  • tasks/archive/2026-02-17-markdown-file-rendering.md
  • tasks/archive/2026-02-17-close-last-terminal-tab.md
  • tasks/archive/2026-02-17-pwa-in-app-navigation.md

Constitution & Risk Check

Checked constitution alignment with focus on Principle XI scope relevance:

• No hardcoded internal deployment URLs were introduced.
• Added polling/retry values are UI-only behavior constants (not API/security/business limits).
• No auth/secrets/credential model changes.

Key tradeoff: git badge polling interval is fixed at 30s in UI for responsiveness vs network churn.