feat(extensions): align static skills and Codex MCP import - #1899

Summary

This combines two independently bounded extension improvements in one delivery PR:

• align static Skill discovery with Claude Code and Codex source semantics through one source-slot parser and one shared argument-expansion owner
• add a fail-closed Codex MCP import projection, with shared final URL validation and minimal CLI/Desktop selection UX
• update the architecture and CLI documentation to describe the supported behavior and explicit non-goals

Design boundaries

• Imported MCP entries remain disabled until the user explicitly enables them.
• Codex servers that depend on environment interpolation, headers, bearer tokens, runtime-sensitive fields, or an implicit workspace working directory are reported as setup-required instead of being copied unsafely.
• Claude Skill metadata that implies execution, hooks, dynamic substitution, or runtime policy is rejected; allowed-tools never grants capabilities.
• No plugin runtime implementation, file watcher, source graph, generic importer, import journal/undo system, new reload command, or product-customization framework is added.

Adversarial review

A fresh-context subagent reviewed the complete diff. No P0/P1 issue remained. The review led to fixes for Codex implicit-cwd handling, selection changes during an active import, runtime-sensitive Skill frontmatter rejection, and stale CLI documentation.

One deliberate low-risk boundary remains: source/scope labels are resolved from the current catalog snapshot. A candidate that appears only after planning can briefly show Unknown; expanding the public plan DTO solely for this presentation edge was rejected as out of scope. The stale candidate still cannot be imported accidentally.

Verification

• cargo check --workspace
• cargo test -p bitfun-agent-runtime --test skill_contracts (29 passed)
• cargo test -p bitfun-services-core --features markdown --test markdown_owner_contracts (9 passed)
• cargo test -p bitfun-product-domains --features external-sources --test external_source_contracts (42 passed)
• cargo test -p bitfun-codex-adapter --test mcp_source (16 passed)
• cargo test -p bitfun-core claude --lib -- --nocapture (5 passed)
• cargo test -p bitfun-cli mcp_import::tests::render_plan -- --nocapture (2 passed)
• focused Web UI tests (18 passed)
• pnpm run type-check:web
• pnpm run lint:web
• node scripts/check-core-boundaries.mjs
• pnpm run check:repo-hygiene
• pnpm run i18n:audit
• pnpm run theme:color-audit:all