Unify assistant auto-capture policy

[jorisjonkers-dev-agents]

Summary

• Add a structured assistant-api capture request carrying provenance, session id, confidence, and duplicate-check metadata.
• Enrich assistant-ui auto-capture candidates with trigger terms, evidence excerpts, confidence tags, and policy details in the captured body.
• Suppress likely duplicate auto-captures via one-hit KB recall before consuming the per-session write budget.
• Route low-confidence assistant captures to _inbox for curator review while keeping high-confidence captures scoped to the inferred project/agent scope.
• Advertise the already-supported source field in the knowledge-api MCP capture tool schema.

Verification

• ./gradlew :services:assistant-api:test :services:knowledge-api:test
• ./gradlew :services:assistant-api:ktlintCheck :services:assistant-api:detekt
• ./gradlew :services:knowledge-api:ktlintCheck :services:knowledge-api:detekt
• git diff --check

Attempted ./gradlew :services:assistant-api:integrationTest :services:knowledge-api:integrationTest; both integration suites are blocked in this local environment because Testcontainers cannot find a Docker socket (/var/run/docker.sock).