feat(tool): add SearchX and ComputerUse tools

[Patel230]

Summary

Adds the two previously-deferred provider/platform-gated capabilities as boundary-compliant hawk tools:

SearchX (internal/tool/search_x.go) — live X/Twitter search. Forwards a query to an xAI endpoint with server-side search (grok-cli pattern), returns a cited summary. Reads XAI_API_KEY with GROK_API_KEY fallback. Inline HTTP (no eyrie/client import), httptest-testable.

ComputerUse (internal/tool/computer_use.go) — desktop automation surface (snapshot/click/type/scroll/press/screenshot) via a pluggable tool.SetComputerBackend seam, mirroring SetMediaEngine. The native macOS accessibility backend is host-wired; the tool reports a clear error until wired.

Both registered in cmd/chat_tools.go (essentialTools) and documented in the README adopted-capabilities table.

Verification

• 12 new tests pass (6 per tool) covering success, error status, missing key/query/backend, invalid action, dispatch
• go build, go vet, full ./internal/tool/ + ./cmd/ suites green
• Boundary guard: zero eyrie/client imports (inline HTTP)