feat(agent): add opt-in live event streaming to AgentBuilder

[niclaslindstedt]

Summary

A zag user reported that migrating from a shell-out-to-zag wrapper (which
tailed the CLI's stderr) to the in-process AgentBuilder::exec().await API
removed the live per-step agent output they used to see in their terminal.
exec captured everything into AgentOutput silently; output.result was
still there for saves/dependency injection, but there was no intermediate
stream. Their migration note said "A follow-up can restore live visibility
by tailing the zag session log in parallel" — and for that plan to work the
library had to start writing a session log when exec runs. It didn't.

This PR makes live visibility a one-line opt-in on the builder and aligns
the library with CLAUDE.md's "libraries are the source of truth" rule.

Library-side usage

let output = AgentBuilder::new()
    .provider("claude")
    .stream_events_to_stderr(ListenFormat::Text)   // <-- the fix
    .exec(prompt).await?;

Or with a custom callback:

AgentBuilder::new()
    .on_log_event(|ev| my_tui.push(ev.clone()))
    .exec(prompt).await?;

Changes

• New builder setters (zag-agent/src/builder.rs):
  • enable_session_log(bool) / session_log(SessionLogMode) — opt-in
  • on_log_event(fn) — generic per-event callback
  • stream_events_to_stderr(ListenFormat) — convenience tail-to-stderr
  • stream_show_thinking(bool) — include reasoning events
• SessionLogMode — Disabled (default) / Auto / External(coord).
  External lets advanced callers (zag-serve, review/plan handlers) hand
  in a pre-built coordinator so we don't double-start.
• Event tap on SessionLogWriter::emit — single chokepoint, fires
  outside the writer mutex so subscribers can't deadlock or block peer
  emitters.
• AgentOutput::log_path — new Option<String> populated when
  logging ran so callers can tail_session_log the file themselves. Marked
  #[serde(default, skip_serializing_if = "Option::is_none")] so all six
  bindings stay JSON-compatible.
• Moved ListenFormat + format_event_* from zag-orch/src/listen.rs
  down into zag-agent/src/listen.rs. Dep direction is zag-orch → zag-agent, so the builder couldn't reach them before. zag-orch::listen
  re-exports the moved items for backward compatibility.
• Moved live_adapter_for_provider from zag-cli into zag-agent
  (the per-provider adapters already lived there).

Backwards compatibility

• Default SessionLogMode for AgentBuilder is Disabled, so existing
  library consumers see no side effects unless they opt in.
• CLI path (AgentFactory + own coordinator) is untouched.
• zag-serve path (AgentFactory direct) is untouched.
• Bindings (TypeScript/Python/C#/Swift/Java/Kotlin) shell out to the zag
  CLI — unaffected. They'll silently ignore the new log_path JSON field.

Test plan

• make build — workspace compiles
• make clippy — zero warnings (cargo clippy --workspace --all-targets -- -D warnings)
• make test — all 1039 tests pass
• New tests in zag-agent/src/mock_integration_tests.rs:
  • test_exec_without_session_log_leaves_log_path_none
  • test_exec_with_enable_session_log_populates_log_path
  • test_on_log_event_receives_lifecycle_events
  • test_on_log_event_implicitly_enables_session_log
  • test_stream_events_to_stderr_implicitly_enables_session_log
• Manual: run zag listen --latest in a second terminal while a
  builder-driven exec is running and confirm the log is identical before
  and after this PR.
• Manual: write a small program that calls
  AgentBuilder::new().provider("claude").stream_events_to_stderr(ListenFormat::Text).exec(...)
  and confirm per-step events appear live on stderr.

https://claude.ai/code/session_019yzV2V34EeL9TJjtw5PaRL