Signature merge action through the agent_action gate (#5632 Phase C C10) #6006

Claude posting for Erik Darling.
Part of #5632 (Phase C, wave 3).

What it does

An MCP action that merges two deadlock signatures, through the Phase B agent_action gate: journaled, with a reason, and undoable. Darling only. Merged signatures share counts; undo splits them. A Level 1 route is refused.

Decisions this slice carries (agreed by Erik on 2026-10-16)

• D14 (A). "An MCP action through the Phase B agent_action gate: journaled, with a reason, undoable. Darling only." (B, operator only in the viewer, was not chosen.) D19 excludes this action from Lite.

Files to touch

Pointers re-checked on dev at c12233ce3.

• Darling/PerformanceMonitor.Darling.Service/Mcp/DarlingAgentActionGate.cs:31 (DarlingAgentActionGate, RunAsync at 41; refusals at 61-63) and Darling/PerformanceMonitor.Darling.Service/Mcp/DarlingMcpAgentActions.cs (the existing gated tools and their undo, lines 180-282).
• PerformanceMonitor.Common/Investigations/InvestigationToolCore.cs:69-120 (agent_action validation).
• The Phase B spine and Agent changes list (Level 2 spine: the agent_action gate and journal #5912, Agent changes list with one-click undo and the switch #5917) already journal and list changes; reuse them so a merge shows in the Agent changes list with one-click undo.
• The merges table is from Phase C store: the one migration, grants and retention (#5632 Phase C C1) #5997; the counts that merge are Deadlock signatures and digest: signature, rollup, gate, digest (#5632 Phase C C9) #6005's rollup.

Depends on

#6005 (signatures and the rollup) and the Phase B gate (#5912, closed).

Behaviour to pin

• Merged signatures share counts.
• Undo splits them again.
• A Level 1 route is refused.
• The merge is journaled with its reason and shows in the Agent changes list.

Tests

Behaviour pins only, one test per behaviour listed above. No census, roster or source-text test (the #5833 ruling in .github/REVIEW-TRAPS.md).

Notes

• Nothing in Lite (D19).
• Nothing under deprecated/.
• At most four parts. If the work needs more, stop and say so in the PR body rather than growing the slice.
• Report the changelog entry in the PR body (the coordinator splices it after the wave; do not edit CHANGELOG.md).