fix(agent): propagate custom-agent request cancellation

[DeliciousBuding]

Summary

Closes #2329
Refs #2256 (only the custom-agent request-context slice; other candidates remain open).

• Bind all six custom-agent CRUD database operations to the supplied request context, without changing the shared GORM handle, service signatures, ownership checks, or Timeout lifecycle synchronization.
• L0 asserts persisted data for canceled create/list/update/delete and cancellation after the ownership SELECT.
• L1 runs the real HTTP handler/service/repository against PostgreSQL with a table lock and a dedicated single-connection pool. Both a deadline and client disconnect must finish server-side work and unblock a queued caller while that lock remains held.

Evidence

• Red before fix: all six new L0 subcases failed; canceled create/update/delete still mutated persisted rows.
• Red L1: after cancellation plus a 2 s observation window, both modes still held the only connection (InUse=1, Idle=0, one queued caller). SQL completed only after fixture cleanup released the lock.
• Green focused L0/L1: canceled mutations, normal CRUD, narrow-update/ownership and structured-output regressions pass. On one Windows/amd64 Go 1.26.5 run with PostgreSQL 16 and Redis 7, observed cancellation-to-release was about 24 ms for the 1 s deadline and 6 ms for client disconnect; HTTP deadline returned 504. These are isolated L1 observations, not production throughput claims.
• Linux/amd64 CGO race: full service/agent, repository, and middleware packages pass.
• Hub go vet and staticcheck 2026.1 pass; doc-SSOT, CI-policy and diff checks pass.
• Full Linux Hub L0 (go test ./... -short -count=1) passes. Full PostgreSQL + Redis L1 with -race passes (33.339 s test runtime); both new cancellation cases also pass under race. Migration 0040 PostgreSQL audit also passes (2 tests). Required CI gates are checked separately before merge.

Environment / not claimed

The local Windows CGO compiler probe fails independently of the changed code; CGO=0 cannot run existing go-sqlite3 tests, so it is not accepted as a full-L0 substitute. Linux CGO/race is used instead and Windows CI remains required. An existing integration helper hard-codes localhost:5432, so a temporary loopback relay targets the disposable fixture for the full local L1 suite. No production or development service, credentials, DNS/ACL, runtime configuration, or deployment was changed. Identity in this test is a fixture; no L3/L4 claim.

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