feat(agents-api): list source files

[SaladDay]

The standalone Agents API could create and retrieve project-owned Files but could not discover them through the pinned client. This adds GET /v1/files with project-scoped (created_at, id) keyset pagination, the fixed SDK query surface, exact list envelopes, and generated SQL/OpenAPI contracts.

Validation:

• fixed openai-python 3.13.0 and raw HTTP against the standalone API and real PostgreSQL, including pagination, filtering, isolation, restart, and cleanup
• focused API/store race tests and SQLC/OpenAPI drift checks
• full make check on zju_a100_2
• fresh blind review of the complete diff: no blocking or substantive findings

Known limits remain documented: exact hosted default ordering, invalid/deleted cursor error parity, and pagination during concurrent mutation are unverified. This resource-only change does not add Runtime, Session, Device, or execution behavior.