fix: skip trail context injection for review sessions
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Commit

- Skip ContextInjector emission for review/investigate sessions (state.Kind != "") so they don't pay the trails-enabled probe or spend context tokens on a branch-tracking hint.
- Make the accepted first-turn tradeoffs explicit in comments: the probe is synchronous so the hint can reach the first model call, bounded by timeout, and transient probe failures fail closed for that session.
- Update the ContextInjector architecture note to match the state-field based once-per-session decision.
Entire-Checkpoint: c27e1c8eb9e9
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Checkpoint 1