agent_synthesis team strategy records a deviations marker only — no synthesis agent is dispatched

[DaveGerson]

Severity: medium · Area: engine / team execution

Capability: SynthesisSpec.strategy = 'agent_synthesis' is supposed to dispatch a synthesis agent to combine team-member outputs after a team step completes.

Documented claim: docs/design-decisions.md previously stated "agent_synthesis (dispatch a synthesis agent to combine outputs)" without a deferral caveat. agent_baton/models/execution.py docstring says "dispatch a synthesis agent to combine outputs." (docs/engine-and-runtime.md already said "(future)".)

Actual state: agent_baton/core/engine/executor.py:5142-5146 only appends a marker string to parent.deviations and sets parent.files_changed. No agent is dispatched, no DISPATCH/INTERACT action is emitted. The internal docstring at executor.py:5117-5120 admits: "the synthesis agent dispatch is deferred to Phase 3.3 (INTERACT action type) which requires invariant changes."

Why it is a stub: Phase 3.3 is unimplemented; _merge_team_member_results() silently falls through with only a marker when agent_synthesis is selected, giving callers concatenate-like behavior.

Suggested fix: Either implement Phase 3.3 (emit a DISPATCH for the synthesis agent after team-member results merge, passing combined context as the delegation prompt), or raise a validation error when a plan is saved with strategy='agent_synthesis' until the feature ships — so callers are not silently downgraded.

Evidence: agent_baton/core/engine/executor.py:5117-5146.

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_{🤖 Filed from the capability-vs-docs audit (HEAD 12999f2). docs/design-decisions.md already updated to mark dispatch as deferred.}