Phase 2 of daemon-owned notebook loading:

- Add `load_notebook_from_disk()` - loads .ipynb file and populates
  NotebookDoc with cells and metadata
- Add `create_empty_notebook()` - creates new notebook with one code
  cell and runtime-specific metadata
- Add `build_new_notebook_metadata()` - builds NotebookMetadataSnapshot
  for new notebooks (Python/Deno, UV/Conda)
- Add `parse_metadata_from_ipynb()` - extracts metadata using existing
  `NotebookMetadataSnapshot::from_metadata_value()`

Reuses existing `parse_cells_from_ipynb()` for cell parsing.
These functions will be called by the OpenNotebook/CreateNotebook
handshake handlers.

Initialize conda notebooks with ["conda-forge"] instead of empty channels
to match the launch logic normalization. Prevents false channel-drift
detection that would trigger unnecessary restart-required behavior.

…ng (#598)

Add new connection methods for the OpenNotebook and CreateNotebook handshakes:

- connect_open_split: Opens existing notebook file via daemon, returns
  NotebookConnectionInfo with notebook_id and trust status

- connect_create_split: Creates new notebook via daemon, returns
  NotebookConnectionInfo with generated notebook_id

Also adds:
- init_open_notebook: Generic init for OpenNotebook handshake
- init_create_notebook: Generic init for CreateNotebook handshake
- do_initial_sync: Extracted common Automerge sync logic

Both Unix and Windows platforms are supported.

Fixes two issues found in review:

1. Race condition: The cell_count check is now done atomically under
   the write lock. Previously, two concurrent opens could both see
   cell_count == 0 (checked with read lock), then both load, duplicating
   cells.

2. Stale room on failure: On load failure, the room is now removed from
   the rooms map before returning. Previously, the room would persist
   with potentially incorrect trust state (computed at room creation),
   which could bypass trust checks on later retries.

The new OpenNotebook/CreateNotebook handshakes return NotebookConnectionInfo
which includes the protocol version. Sending ProtocolCapabilities again
after that would break the client which expects AutomergeSync frames next.

Added skip_capabilities parameter to handle_notebook_sync_connection:
- false for legacy NotebookSync handshake (sends ProtocolCapabilities)
- true for OpenNotebook/CreateNotebook (protocol already in response)

1. CreateNotebook: Clean up room on failure for consistency with
   OpenNotebook (prevents stale UUID-keyed rooms from leaking)

2. Client: Add protocol version validation for NotebookConnectionInfo
   in init_open_notebook and init_create_notebook (defensive check)

3. Tests: Add unit tests for build_new_notebook_metadata (3 branches:
   deno, python-uv, python-conda) and create_empty_notebook (python,
   deno, custom env_id)

The sync loop duplication between init and do_initial_sync is noted
for follow-up refactoring but not addressed here.