Run bounded project coding tasks in owned worktrees - #10658

Project tasks can now run from the project page through the durable engine and the shared Studio inference loop. A coordinator delegates bounded reviewer/implementer children; every attempt receives an owned Git worktree. The panel exposes status, cancellation, explicit retry, parent/attempt relationships, output and a bounded diff against the starting commit, including new files. Changes remain available for human review.

This PR now includes the durable task engine from #10655 together with its first execution/UI consumer. The engine-only PR is retained as a draft reference, and its duplicate standalone workflow is removed here. Remaining prerequisites are #10633 (lifecycle), #10577 (secure edits), and #10594 (Git/worktrees). CI first checks the standalone source's unavailable-prerequisite response, then composes the exact revisions documented in studio/docs/project-task-integration.md. The lifecycle and Git PRs carry optional task hooks; this PR checks their protocols before admitting tasks.

The runtime snapshot binds the model, provider routing and encrypted credential-row version. Tasks never load a model or substitute a provider. Each model turn respects the attempt allocation and saved provider output-token cap, then releases admission before tools or child waits; local admission retains the aggregate KV budget. A private task-only callback restricts coordinator/reviewer roles to reads and implementers to confined, expected-content edits. Tool results use the shared model-visible cap. Task reads, listings, waits and optional verification commands can repeat identical arguments to observe changed state; ordinary-chat deduplication, one-shot restrictions and all task budgets remain enforced.

Failed setup rolls back a newly created, provably clean owned checkout before execution; dirty or ambiguous checkouts remain in Git recovery. Polling batches worktree bindings in one query. Incompatible prerequisite versions return 503 and permit shutdown. Native project/worktree fences survive revoked database leases and remain held until physical tool workers return. Retirement drains tasks before acquiring Git's exclusive fence.

Optional #10672 command hooks add a default-off capability checkbox, captured opt-in and bounded command-evidence views. Without that separate source layer and a supported Linux supervisor, the capability stays unavailable. Task completion and command success are presented separately. This integration layer does not itself execute commands.

Scope: owned worktrees require POSIX; runtimes are saved tool-capable providers accepting output caps or an already loaded matching GGUF runtime with idle-slot clearing. Subscription endpoints without output caps, safetensors execution, automatic commits/merges, cleanup of bound task worktrees and existing-folder adoption remain outside this layer. Children start from the captured commit. No live-model/GPU quality or performance claim.

Validation:

• The actual shared tool loop, SQLite engine, and Git worktrees run with deterministic simulated model responses. Local composed validation passed 321 cases with six platform skips. The standalone prerequisite gate passed 19 cases with 28 prerequisite-dependent skips.
• All 7,293 frontend tests, app/test typechecks, production build, and startup budget passed locally. The branch uses upstream's existing 1,645,000-byte transfer and 5,500,000-byte raw startup limits; task UI/API/review code remains lazy.
• Integration CI 34482894071 passed all four jobs at source head c27726c496531dcfc59b7a039f1cc05dee74b8c0: Linux, macOS, Windows, and frontend qualification. CI composes lifecycle 10eec87e4, confined edits a85723e1c, and Git/worktrees 9d86e2547.

Current head 94baadc4c42ec88fbf610ac0fa8aefce4b6a0c5a changes only the review guide to match those dependency pins and the upstream budget. All executable source, tests, and workflows are identical to the qualified source head. Maintainer review and prerequisite merges remain separate gates; this does not satisfy #10672's requested rebase onto merged main-branch foundations.