MAG-229 fix Redact agent env

[tasu64]

Summary

• redact agent custom_env values from agent list/get and historical task-message readback
• preserve routing metadata while exposing only custom_env coarse metadata
• make the DB-backed historical readback regression execute by updating handler test schema setup

Verification

• go test ./pkg/redact -count=1
• go test ./internal/handler -run 'Test(GetAgent_ResponseHasNoCustomEnv|ListAgents_ResponseHasNoCustomEnv|ListTaskMessagesByUser_RedactsHistoricalAgentCustomEnv|ListTaskMessagesByUser_InvalidTaskIDReturnsBadRequest)' -count=1 -v
• pnpm --dir packages/views exec vitest run common/task-transcript/redact.test.ts common/task-transcript/build-timeline.test.ts
• pnpm --dir packages/views typecheck
• git diff --check

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