refactor(container): report lifecycle phases as structured events

[jonathanKingston]

Container lifecycle state depended on matching human-readable diagnostic strings. Replace those matches with typed worker phase records, decoded at the pipe boundary and forwarded through an explicit runner callback. Ordinary logs no longer advance state, and the guest cannot declare a terminal outcome.

Follow-up to #2348, targeting its feature branch so this PR contains only the phase change. Also validated in an isolated checkout rebased onto origin/main at 4253dfd0f.

Validation: typecheck, lint, formatting, dead-code, oracle, and e2e-syntax checks passed; 48 focused tests passed, including split pipe writes and malformed/terminal records. The full sandboxed suite reported 8,824 passing, 42 failing, 41 cancelled, and 9 skipped tests; failures include denied loopback/Unix socket listeners (EPERM). Existing phase labels and DOM are unchanged. All four changes were also integrated: 8,953 full-suite unit tests passed outside the sandbox, and the final 208-test targeted regression run passed after the additional cancellation fix.

[github-actions]

🖥️ PR preview

• Marketing site
• Browser demo
  Interactive, with a mocked backend — jump to a scenario:
• Builds a cupcake site
• Agent-proposed edits open the Changes panel
• Markdown list indentation
• Unattended container run
• Responsive composer footer
• Subagent display visual reference
• Subagent stays ungrouped beside reading tools
• Semantic search subagent markdown
• Light-theme approval with a bright accent
• Light-theme syntax, fills, and selection
• Grouped outside-sandbox command approval
• Remote desktop discovered-port list with one selected
• Settings scroll + sticky footer geometry
• Per-model reasoning / temperature / top-p
• Thread switch hydration notice
• Thread switch hydration failure
• Chat layout styling
  _{Static demo of the renderer against a mock API — the browser pane, terminal, and all main-process IPC are stubbed, so it never proves main-process behaviour. Rebuilt whenever a push changes the demo bundle; these links go live once the queued deploy lands, shortly after this comment. Removed when the PR closes.}