fix(mcp): remember tool output validation — schema vs handler mismatches

[cdeust]

Summary

The remember MCP tool failed output validation on every code path due to schema/handler mismatches. Three independent fixes in one commit:

1. memory_id / merged_with type — schema declared string (UUID-style) but the handler returns int (PG bigint). Now integer, aligning with anchor.py / forget.py / get_causal_chain.py which already declare integer.
2. Rejection paths missing action — schema invariant required: ["stored","action"] was violated by both no_content early-exits in remember.py and by write_gate.build_rejection_response(). Added "action": "rejected" to each.
3. Action enum mismatch — schema enum was ["stored","merged","rejected"] but the handler forwarded internal curation vocab "create" / "link" untouched, so every successful create/link failed with 'create' is not one of [...]. Normalized at the response boundary in build_response: create/link → stored, merge → merged.

Repro (before)

cortex:recall    → Output validation error: 'memories' is a required property  (separate bug, already fixed in trunk schema)
cortex:remember  → Output validation error: 'NNNN' is not of type 'string'
cortex:remember  → Output validation error: 'action' is a required property
cortex:remember  → Output validation error: 'create' is not one of ['stored', 'merged', 'rejected']

After this PR, the schema invariant required:["stored","action"] and the enum constraint are honoured on every code path (no_content, gate-rejected, created, linked, merged).

Test plan

• Restart Cortex MCP server
• Call remember({content: "test memory"}) — expect {stored: true, memory_id: <int>, action: "stored", ...}
• Call remember({content: ""}) — expect {stored: false, action: "rejected", reason: "no_content"}
• Call remember({content: "<duplicate of existing>"}) — expect {stored: false, action: "rejected", reason: "<gate reason>", ...} from write gate
• Call remember({content: "<very similar to existing>", force: false}) — expect {stored: true, action: "merged", merged_with: <int>, ...}
• All four responses pass MCP output schema validation (no Output validation error)

Files

• mcp_server/handlers/remember.py — schema types + early-exit action
• mcp_server/handlers/remember_response.py — build_response normalization
• mcp_server/core/write_gate.py — build_rejection_response action field

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