fix(core): fix two broken reaction sequences (closes #614)

[marcpineault]

Summary

Audit of the reaction engine found two bugs that together produce the double-failure described in #614: the agent never gets the CI error AND the human never gets notified.

• send-to-agent silent no-op when message is missing — When a reaction config omits the message field (as in the documented examples/auto-merge.yaml), the send-to-agent case skipped the send entirely and returned success: false with no log, no notification, and no indication anything went wrong. Added a static fallback message map for the five built-in send-to-agent reaction keys so agents always receive actionable guidance. Added reaction.execute observability traces for failed sends and the no-message/no-fallback edge case.

• reactionHandledNotify suppressed human fallback even on failure — reactionHandledNotify was set to true unconditionally after executing any reaction, even when the reaction returned success: false (e.g. agent unreachable, send threw). This silently dropped the event with no fallback. Changed to reactionHandledNotify = result.success so a failed reaction still triggers the human notification path.

Test plan

• New test: send-to-agent without message uses fallback (verifies exact "CI is failing" string)
• New test: failed send-to-agent (throws) still fires human notification
• Existing 30 lifecycle-manager tests still pass (32 total, no regressions)
• pnpm build clean
• pnpm typecheck clean
• No lint errors in changed files

🤖 Generated with Claude Code

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