Add PutBlob frame to runtime-agent ↔ daemon socket (#1334 dep)

[rgbkrk]

Currently the runtime agent writes blobs through the daemon's separate blob channel connection. Add a PutBlob frame type on the existing runtime-agent sync socket so:

1. Only one persistent connection per agent (we already have it for sync + requests)
2. Remove the separate blob channel's handshake path
3. Simplifies kernel-launch blob-store permissions

Current frame types in crates/notebook-doc/src/frame_types.rs: 0x00 AutomergeSync, 0x01 Request, 0x02 Response, 0x03 Broadcast, 0x04 Presence, 0x05 RuntimeStateSync, 0x06 PoolStateSync. Add 0x07 PutBlob or similar.

Wire shape

| 4 bytes length | 1 byte type (0x07) | 32 bytes hash | rest is blob bytes |

Or make the payload JSON-prefixed so we can extend (hash, content_type, size) cleanly.

Related

• feat(runtimed): SSH remote runtimes #1334 — dependency for the agent-side blob upload path
• contributing/protocol.md for where this documents