feat(extensions): complete MCP, Plugin and LSP onboarding

[Qiyuanqiii]

Summary

Settings can now complete first-time MCP, Plugin and LSP setup through the shared Go control plane. Users can select a Workspace before a Run exists, register a manual MCP descriptor, upload a pinned Plugin ZIP, or stage an installed language server. Forms retain input after failure, show source/scope/review metadata and provide the next available action. Web/Desktop use the same backend-advertised onboarding capabilities and generated API contract.

Registration stays inert. MCP retains separate discovery and capability reviews; actual calls use the ordinary task/Approval/Gateway path and settings reads their durable audit result. Plugin import retains the existing 4 MiB plugin.v1 validator and explicit package/capability reviews. LSP review persists the existing operator configuration format; a separate bounded symbol-query test returns real server/query/document provenance. A stable manager connects new configurations to model tools, rejects stale queries and owns process cleanup. OS advisory locking and digest checks prevent concurrent configuration overwrites. Explicit CLI/environment LSP files remain authoritative and read-only in settings.

Plugin upload preflight uses bounded decoding and canonical re-encoding so supported 3.5–4 MiB payloads do not overflow the browser regex stack. Both encoded-length and decoded-byte limits remain enforced, and malformed or noncanonical input is rejected before the request.

The application preview copy now describes the available service/log view separately from browser availability.

The existing real Edge CI job uses lowercase nul for its empty Git configuration, accommodating the Git for Windows 2.56/UCRT regression (upstream #6449). Source-binding failures now retain Git stderr without mixing it into the source metadata.

Closes #276.

Validation

• Full CI run 37566854056 is running on merge commit 0955d7fc0aab7539cdb0fbb0b8930582ae9c7c92, synchronized with main 7aac2773451daff1e372647a5733c937c00d2cc7. GitHub reports the PR as mergeable.
• Complete frontend suite: 174 files / 1,639 tests pass, including task search, execution labels, onboarding and all 14 Plugin Base64 boundary regressions. Production build and npm run check:api pass.
• Complete HTTP package (197.68s) and domain package (0.16s) pass. Existing Store title-search and execution-fact regressions pass (2.76s).
• HTTP coverage includes inert MCP registration and separate review, normal task/approval/Gateway invocation and replay, Plugin import/review/Hook receipts, real LSP setup/query/restart, concurrent configuration owners and rejected scope/hash/authentication changes. The shared MCP product test also passes with task-list approval and cancellation projections.
• Go OpenAPI and TypeScript snapshots were regenerated and exactly match the merged contracts. Protocol and Surface registry checks pass against main 7aac2773451daff1e372647a5733c937c00d2cc7.
• go vet for HTTP, application and Store packages, and git diff --check, pass.
• Earlier browser evidence for the unchanged Base64 helper: Chromium 154.0.8037.98 reproduced the grouped-regex stack overflow and passed all 14 cases after the fix on 395d7bfc. This is a browser-side preflight probe, not a complete import/install test of a large ZIP. The supported 3.5 MiB, 4 MiB minus one byte and exactly 4 MiB client requests are covered again by the current full frontend suite.
• This change uses the existing applicable CI and introduces no new workflow or duplicate full-suite gate.

Surface governance

• No Surface is added, promoted, downgraded, deprecated, or removed.
• Registry declarations: N/A — no Surface change. Existing optional extension integrations remain behind Go-owned control authentication, review, policy and scope. Existing CLI flows remain available.

Audit

• No credentials or local runtime data are included.
• Policy, workspace, process ownership and persistence boundaries were reviewed. Public inventories exclude LSP launch arguments, executable paths, private roots and raw output. The explicit operator LSP probe starts only a reviewed selected server; it does not widen model authority or bypass the ordinary MCP call path.
• Usage, code-intelligence documentation, architecture and project memory are updated; ADR 0168 records lifecycle, review, compatibility and rollback behavior. No SQLite migration is added.