feat: unify external extension conflict management

[limityan]

Summary

• keep pending and resolved Command, Tool, and Subagent name conflicts visible and changeable in Desktop settings
• distinguish currently used from selected but currently unavailable, including restricted commands, disabled tools, and disabled external agents
• consolidate TUI management into the existing Tools and Agents surfaces, remove parallel extension commands, and provide migration guidance for removed commands
• keep Agent management available during an active turn while rechecking the live turn state before switching the main Agent
• let a missing Node.js path recover after refresh without replacing an active runtime, and fail closed when BitFun model settings cannot be read

Product and architecture behavior

• BitFun/native candidates remain first in conflict choices, but no external candidate is silently replaced
• opening management from the command palette or using a management subcommand does not rewrite a remembered external Prompt Command choice
• management actions use the existing /builtin: qualifier only as the collision-safe form; no new top-level slash command is introduced
• GUI and TUI preserve resolved choices, covered candidates, disabled choices, and actionable recovery details
• Remote workspaces continue to reject unsupported local external-source fallback
• Node.js recovery copy recommends install/repair and refresh without forcing or orchestrating a restart

Scope boundaries

• no new extension type, runtime, watcher, telemetry platform, restart orchestration, or disk cache
• no system prompt changes
• no change to the existing Skill discovery or priority order
• ecosystem-specific behavior remains in its adapter/owner rather than the shared management surfaces

Validation

Check	Result
cargo +1.95.0 test -p bitfun-cli	244 unit tests plus all CLI integration tests passed
focused external-source Web test	17/17 passed
pnpm run type-check:web	passed
pnpm run i18n:audit	passed with 0 warnings
cargo +1.95.0 check --workspace	passed
focused external Subagent conflict test	passed
focused Node.js runtime recovery test	passed
core boundary and repository hygiene checks	passed
pnpm run lint:web	0 errors; 12 existing warnings outside the changed files
git diff --check	passed

Independent review

Independent adversarial architecture and product reviews were completed after the fixes. Neither review found a remaining P0-P2 issue or submission blocker.