feat(extensions): support OpenCode MCP configuration sources

[limityan]

Summary

• Discover the supported OpenCode MCP configuration subset from user, explicit config, explicit config directory, and workspace JSON/JSONC sources.
• Convert ecosystem-specific input through the OpenCode adapter into source-neutral MCP contracts and let the existing MCP owner manage connections, tools, status, and retirement.
• Expose the same approval, conflict, status, and recovery model in Desktop settings and the existing TUI /mcps flow without adding product-specific slash commands or requiring a BitFun restart.

Architecture and boundaries

Owner	Responsibility in this change
OpenCode adapter	Source order, JSON/JSONC parsing, safe previews, supported-field conversion, and OpenCode-specific degradation
External-source contracts/coordinator	Provider isolation, generations, last-success behavior, watcher roots, and source-neutral MCP candidates
Product assembly	Native-first conflict policy, one-time behavior approval, workspace route selection, and shared product state
MCP runtime owner	Process/connection lifecycle, catalog publication, tool registration, bounded retirement, status, and cleanup
GUI/TUI surfaces	Present the shared state and invoke shared decisions; neither surface interprets OpenCode configuration

Core-boundary rules cover the new contracts and prevent OpenCode parsing or concrete MCP runtime ownership from leaking into the wrong layer.

Product behavior

• BitFun-native MCP remains the default winner. External conflicts require an explicit choice, and unchanged declined/approved behavior is not asked again.
• GUI conflict resolution uses a review-then-approve flow and shows source, scope, transport, command/origin, environment/header names, and security impact before acceptance.
• TUI keeps the existing /mcps entry, labels native and external sources in one list, preserves selection by stable ID, and supports scrollable approval on 40x20 terminals with Unicode-width-aware wrapping.
• Startup, zero-tool success, unavailable, disabled, unsupported, and removed states are distinct. Recovery for a failed startup is explicit: disable, repair configuration or authentication, then enable; normal refresh does not create an automatic retry loop.
• No BitFun restart is required, and retirement withdraws new tool routes without interrupting unrelated sessions.
• Remote workspaces fail closed and do not fall back to local OpenCode configuration.

Security, reliability, and performance

• External tools are authorized against the current local workspace route on every call. Unscoped Desktop Resource/Prompt/MCP App APIs cannot reuse external runtime IDs.
• External server requests for roots, sampling, elicitation, and other host capabilities fail closed; this slice integrates external MCP Tools only.
• Local child processes inherit only a minimal system baseline plus explicitly declared environment values. Environment names and HTTPS origins are previewed without exposing values or full URLs.
• Behavior-changing command, arguments, working directory, environment declarations, origin, headers, or authentication require a new approval. Expanded values are validated again.
• Per-start tokens prevent stale startup callbacks from publishing or deleting a newer instance with the same deterministic runtime ID.
• Discovery is bounded at 5 seconds, external startup and retirement grace at 30 seconds, status reads at 100 ms, configuration files at 1 MiB, and expanded runtime text at 64 KiB.

Explicitly deferred

• Remote-workspace execution ownership
• SSE transport and complete OpenCode OAuth client configuration
• Full timeout and Agent-scope semantics
• MCP Resource, Prompt, MCP App, roots, sampling, and elicitation integration
• Removed-item tombstone notifications and broader package/plugin support

These remain unsupported or fail closed rather than being partially executed.

Validation

• cargo check --workspace
• OpenCode MCP adapter: 5 tests
• External MCP coordinator: 3 tests
• External-source contracts: 19 tests
• MCP service contracts with the mcp feature: 38 tests
• Core external MCP/status/security/start-token lifecycle: 10 focused tests
• TUI MCP selector: 6 tests
• Web external-source settings: 18 tests
• pnpm run type-check:web
• pnpm run i18n:contract:test (37 tests)
• pnpm run i18n:audit (0 warnings)
• Core-boundary self-test and repository boundary check
• pnpm run check:repo-hygiene
• git diff --check

The Rust build still reports existing MCP dependency deprecation warnings and existing CLI dead-code warnings; no new warning category was introduced by this change.