feat: chunked exec streaming — server-side temp file assembly (#116)

[blaspat]

What

Adds server-side chunked exec streaming support — receives exec_chunk messages from nodes, assembles base64-encoded chunks into temp files, and resolves waiters with the file path on completion.

How

Config — four new fields on NodeServerConfig:

• exec_chunk_dir — temp file directory (default /tmp/hermes-exec-chunks)
• exec_chunk_max_total_mb — total disk cap across all active chunks (default 50 MiB, 0 = unlimited)
• exec_chunk_timeout_s — stale chunk timeout (default 30s)
• exec_chunk_size — advisory chunk size advertised to nodes (default 64 KiB)

Registry — ChunkState dataclass + lifecycle methods (get/set/remove_chunk_state, fail_node_chunks). Wired into register()/unregister() for disconnect cleanup.

Server dispatch — _handle_exec_chunk() intercepts exec_chunk messages in the WebSocket loop, decodes base64, appends to temp files via mkstemp, resolves waiters with {output_file, exit_code, status, duration_ms, truncated} on final chunk.

Cleanup:

• Timeout sweep (background task, 5s interval) — closes files + deletes temp files + fails waiters
• Startup orphan cleanup — deletes leftover *.chunk files from unclean shutdown
• Disconnect cleanup — fail_node_chunks on node disconnect/replace

Tools — _node_exec_impl dual-path read: output_file (chunked) vs legacy stdout/stderr.

hello_ack — advertises exec_chunk capability so nodes know the server supports it.

Tests: 46/46 pass.