fix(agent-config): refresh agent registry on config apply

[SpencerJung]

Summary

• Reset the registered agent-name lookup before each config-agent registration pass.
• Re-register the freshly rebuilt agent config so /connect config re-entry cannot leave stale selector aliases behind.
• Add regression coverage for clearing aliases and for config re-application keeping OMO agents registered.

Fixes #4130

Verification

• lsp_diagnostics on src/features/claude-code-session-state/state.ts
• lsp_diagnostics on src/plugin-handlers/agent-config-handler.ts
• lsp_diagnostics on src/features/claude-code-session-state/state.test.ts
• lsp_diagnostics on src/plugin-handlers/agent-config-handler.test.ts
• bun test src/features/claude-code-session-state/state.test.ts src/plugin-handlers/agent-config-handler.test.ts
• bun run typecheck
• bun run build
• git diff --check

---

Summary by cubic

Refreshes the agent registry on each config apply so /connect can’t leave stale agent aliases behind. Keeps the registry aligned with the freshly rebuilt config and adds regression tests.

• Bug Fixes
  • Clear registered agent names and aliases before re-registering from the current config.
  • Ensure current/builtin agents are re-registered; stale names are removed.
  • Add tests that verify alias clearing without losing session ownership and that re-applying config replaces stale registrations.

^{Written for commit 66fefc3. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 4 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Auto-approved: The PR addresses a specific bug by clearing stale agent aliases before re-registering from fresh config, with targeted changes that have no apparent side effects and are covered by regression tests.

Re-trigger cubic</sub>