refactor(container): track external runs with the task supervisor

[jonathanKingston]

Container runs had separate, session-only lifecycle ownership. Register them as external tasks with the existing supervisor before image preparation, persist ownership and terminal outcomes, route cancellation through its external canceller, and cancel live runs during app shutdown. One AbortController replaces the separate stop-request ledger, including stops during checkout preparation.

Docker execution, attestation, credentials, egress, carry-out and runtime budget enforcement remain in the runner. External task adoption does not enforce queued-handler concurrency limits. Lost runtime handles fail on restart; no handler replays the run and explicit continuation remains required. Prompts and credentials are not stored in supervisor metadata.

Follow-up to #2348, targeting its feature branch. Contract documented in docs/plans/thread-in-container.md.

Validation: 61 focused lifecycle/reconciliation tests passed; the final 34-test service run also passed, including the regression for starting a new run while the previous outcome is still being saved; all 207 combined regression tests passed outside the sandbox. Typecheck, lint, formatting and dead-code checks passed in an isolated checkout rebased onto origin/main at 4253dfd0f; all static check stages passed. The integrated full unit suite passed 8,953 tests outside the sandbox, and the final targeted run passed all 208 tests after adding the cancellation regression. Redundant sandboxed full-suite runs were stopped. Build passed. Focused Electron e2e (container-supervised-task.e2e.ts) passed; screenshot inspected: readable container task label, blocked state and cancel action in the Background tasks rail, then row removal after cancellation. Screenshot included.

[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.}