feat(events): emit GOAL_ACHIEVED on terminal-node success

[gavksingh]

Summary

• Add emit_goal_achieved() convenience method to EventBus, mirroring the pattern of existing emitters like emit_constraint_violation()
• Emit GOAL_ACHIEVED from GraphExecutor when execution completes at a terminal node successfully
• Update EVENT_TYPES.md to document the payload shape and emission point
• Add unit + integration tests for the new emission

The GOAL_ACHIEVED EventType was already defined in the enum and fully handled by the TUI (subscription, chat_repl handler, log_pane formatting), but no backend code ever emitted it. This completes the agent lifecycle observability story.

Fixes #5736

Test plan

• emit_goal_achieved() publishes an AgentEvent with correct type, stream_id, execution_id, and data
• Default output is {} when None is passed
• GraphExecutor calls emit_goal_achieved on successful terminal-node completion
• GraphExecutor works without crash when event_bus is None
• Existing test_executor_skips_events_for_event_loop_nodes updated to distinguish node-level vs graph-level events
• All 774 existing tests pass (no regressions)
• ruff check passes on all modified files

[bryanadenhq]

Thanks for wiring up the GOAL_ACHIEVED event, that part looks clean and straightforward. However, this PR bundles in significant fan-out executor changes (memory conflict detection, per-branch timeouts, refactored result processing) that aren't related to the goal achieved event or #5736. Could you split those into a separate PR? It'll make both easier to review and safer to merge/revert independently.

[gavksingh]

@bryanadenhq thanks for pointing that out! The executor changes (branch timeout + memory conflict) were from #5706 which I also worked on and got merged separately via PR #6504. Forgot to rebase this branch after that. Rebased and force pushed now so this PR only has the GOAL_ACHIEVED event emission.