[griffithtp]

Summary

• Fix: widgets showed "no agent connection" while the Desktop Agent was running. With no fdc3 section in terminal.config.json, the Terminal injected an empty agent URL, which fdc3-plugin reads as "FDC3 off". A missing key now falls back to ws://localhost:7891/fdc3. "" still turns FDC3 off. Error messages now name the config key to set.
• Desktop Agent per-user config: agent.config.json in the user's config dir (e.g. ~/Library/Application Support/one-terminal/) layers between the bundled file and OT_* env vars. Partial files are allowed, unknown keys are warned about, and every setting is logged at startup with its source.
• Terminal agent selection:
  • Configure agents with fdc3.agents[] and fdc3.defaultAgent. Users switch from App Menu → Desktop Agent, the header badge, or the command palette.
  • The choice is saved and kept across restarts. OT_FDC3_AGENT_URL overrides it and makes the picker read-only.
  • Switching recreates panel webviews in place, with no restart, and channels are kept.
  • OpenFin, interop.io and Finsemble agents are listed but disabled until their connectors exist.
• Docs: new docs/fdc3.md covering how widgets connect, choosing an agent, connector kinds, and Desktop Agent config. Code comments and the README point to it.
• Broker: implemented GetCurrentContext, FindIntent, FindIntentsForContext, GetRegisteredApps and intent raising in the Desktop Agent and ot-fdc3.
• Scaffolder: migration 0.1.14.

No breaking config changes.

Test plan

• cargo test (12 new tests), clippy, tsc, ESLint and vite build are clean
• Smoke-tested Desktop Agent and Terminal startup with a scratch HOME: each setting showed the right source, and a saved agent that was removed from config fell back to the default with a notice
• Manual: switch agents with several widgets open (stacked and kept-alive) and confirm they all reconnect with channels kept
• Manual: restart and confirm the choice is kept; with OT_FDC3_AGENT_URL set, the picker is read-only