Postmaster v9.3.0 native file resource handoff

[the-code-learner]

Summary

• add get_stored_file_resource(file_id, transport) returning a real MCP ResourceLink
• register postmaster://files/{file_id} for MCP resources/read
• add signed GET/HEAD /files/{file_id} streaming with HMAC expiry and byte ranges
• preserve the v9.2 native ChatGPT upload path and generic Base64 fallback unchanged
• keep file_id canonical and stream the existing SHA-256 FileStore blob without transcode/recompression/temp copies
• add v9.3 regression coverage and docs/FILE_HANDOFF.md
• bump VERSION/CHANGELOG to 9.3.0 and add stable release automation

Deployment

postmaster-mcp.yml remains unchanged. The HTTP handoff reuses existing PUBLIC_MCP_HOST; optional process-level overrides are FILE_STORE_PUBLIC_BASE_URL, FILE_STORE_DOWNLOAD_SECRET, and FILE_STORE_DOWNLOAD_URL_TTL_SECONDS. If no public base is configured, transport=auto falls back to the MCP resource URI. The download secret is generated persistently under /data when not supplied.

An existing POSTMASTER_VERSION=latest deployment can receive v9.3 by restarting after the stable release is published.