release: v0.18.0

Minor bump from 0.17.2. Notable changes since the last release:

Features
- websearch: SearXNG JSON engine as an escape hatch when DuckDuckGo/Bing
  are blocked by TLS fingerprinting / IP reputation (#270).
- mcp: advertise cached tool schemas at spawn with connect-on-first-call,
  so MCP tools are usable from the first turn with no prompt-cache miss in
  the warm case (#206 Phase 2); late-registration refresh is now one-shot
  and surfaced in the UI/logs.
- relay: opt-in headed launch listener.

Fixes
- mcp: surface MCP tools that register after the tool snapshot locks (#206).
- ambient: accept string-encoded numbers/bools in tool inputs (#106); stop
  hallucinated tool names and recover from unknown tools (#104).
- websearch: detect anti-bot/captcha pages and surface a clear error.
- provider: Mistral 422 (#261), configurable SSE idle timeout (#196),
  treat HTTP 402 as non-retryable, OpenRouter provider stickiness.
- server: graceful `server reload`/`server stop`, stale-socket reaping,
  directional newer-binary check (#291).
- tui: prevent phantom chat scroll offset; preserve input when cycling
  model favorites.
- swarm: inherit coordinator model by default; auto-submit headed-spawn
  startup prompt.