Add structured ReACT event sink for protocol adapters (#156)

[jamiesun]

Summary

Adds an opt-in, in-process structured ReACT event channel so protocol adapters and observability sinks no longer have to parse formatted trace lines or poll the audit JSONL. Closes #156. Also lands a small audit-driven CI hardening step.

Foundation for the ACP adapter epic (#155) and the observability hook sink (#137); follow-up to the agent event callback requested in #134.

Changes

feat: structured event sink (#156)

• src/agent.zig — new pub const Event union (thinking, step, policy_deny, running, observation, final, malformed, compacted) and a lightweight pub const EventSink vtable. Added Agent.events and an inline fn emitEvent. Events fire at the top of each trace* method, so they are independent of the text trace writer (fire even with no writer attached) and leave trace output byte-identical. tracePolicyDeny/traceObservation now take the current action.
• src/api.zig — RunOptions.event_sink is forwarded into the agent in runDetailedWithOptions.
• src/root.zig — unchanged; the public-API whitelist stays narrow (RunOptions is internal-only).

Lifetime contract: event slices are turn-arena lifetime, valid only during the synchronous emit call. Sinks must consume immediately and must not retain references (documented on Event).

ci: secret hygiene guard

• .github/workflows/ci.yml — fails CI if config.toml stops being gitignored or if an inline sk-… / api_key="…" literal lands in src/ or config.example.toml. Locks in the current audit-clean state.

Acceptance criteria (#156)

• agent.Event / agent.EventSink exist; Agent.events field present
• All existing trace tests pass unchanged (byte-identical text output)
• Capturing test sink receives the expected ordered events for a scripted run (thinking → step → running → observation → … → final), including a policy-deny path
• api.RunOptions.event_sink forwards into the agent; covered by an api.zig test using TestBrain
• src/root.zig unchanged; public-API whitelist test green
• zig build + zig build test green

Verification

• zig fmt --check clean
• zig build ✅ · zig build test 704/704 passed ✅ · zig build -Doptimize=ReleaseSafe ✅ · --version → scoot 0.5.0

Blast radius

Additive in-process channel; no new execution surface, no behavior change when events == null. Touches src/agent.zig and src/api.zig only (plus the CI workflow).