fix(codex): discover symlinked skills and reload MCP

[fsioni]

What changed

• discover Codex skills installed as symlinked directories while preserving physical and nested skill discovery
• avoid recursively following symlinked categories, preventing directory cycles
• reload the persistent Codex app-server MCP registry before every thread start or resume
• fail the turn explicitly if the MCP registry cannot be refreshed instead of continuing with stale tools
• add regression coverage for symlinked skills, symlink cycles, OAuth HTTP MCP discovery, and the app-server reload protocol

Why

This fixes a regression introduced by #666 (feat(skills): discover skills in nested category directories). That change added recursive category discovery, but also explicitly rejected every symlinked directory and added a test asserting that symlinks must not be followed.

Codex CLI supports skill installations such as ~/.agents/skills/feature-flow -> ~/.claude/skills/feature-flow. After #666, Jean could still receive metadata for such a skill through explicit prompt injection, but excluded it from the invocable skill registry. Physical skill directories continued to work, which made the regression specific to symlink-based installations.

This PR restores Codex-compatible directory-symlink discovery without reverting the useful nested-category support from #666: only symlinks that directly resolve to a skill root are accepted, while symlinked categories are not traversed recursively.

Separately, Jean keeps codex app-server alive across sessions. Its MCP registry could therefore predate changes to ~/.codex/config.toml, causing configured servers such as ClickUp to be absent even though a fresh codex mcp list reported them as enabled.

Validation

• cargo test --manifest-path jean-core/Cargo.toml — 1080 passed, 1 ignored on upstream main
• targeted rustfmt --check for all changed Rust files
• manual Codex app-server protocol check: config/mcpServer/reload succeeded and mcpServerStatus/list included ClickUp
• manual Jean dev smoke test for symlinked skill discovery

bun run check:all currently stops on the existing unrelated lint error in src/components/chat/ChatInput.test.tsx:37 (@typescript-eslint/consistent-type-imports).

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