feat: surface session fork in the TUI (/fork [backend])

[saucam]

What

Adds the /fork slash command to the Rust TUI, so the terminal UI can branch the focused session into an independent one — optionally continuing it on a different backend. This is the third UI to expose fork (telegram + web already shipped in the codeoid repo's feat/fork-provider-ui).

• /fork — branch the focused session, keeping the parent's backend.
• /fork <backend> — branch and continue on another backend (e.g. /fork codex).

On success the fork is added to the tab strip and focused so its tab goes active; the daemon validates fail-closed (unknown session → not_found, unknown backend → invalid_request) and the error surfaces inline.

Changes

• protocol (codeoid-protocol): add ClientMessage::SessionFork { session_id, name?, provider_id? } mirroring the daemon's session.fork wire shape; register it in request_id and client_kind.
• commands: parse /fork and /fork <backend> (lowercased); add a CATALOG entry for autocomplete/help. Drop the now-conflicting fork alias for /import — /import keeps its own name.
• app: fork_focused sends the fork frame and, on success, adds + focuses the returned session. Response-handling is extracted into the pure, unit-tested apply_fork_response.

Tests

• parser: with/without backend, catalog presence
• apply_fork_response: add-and-focus on a valid payload; error on missing/unparseable payload (parent focus untouched)
• client_kind fork mapping; session_fork_message wire-shape
• guard paths: no focused session, no live connection

cargo test --workspace (258 tui tests pass), cargo fmt --all --check, cargo build --workspace all clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 83.91608% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.24%. Comparing base (c519852) to head (f187098).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/codeoid-tui/src/app.rs	81.35%	22 Missing ⚠️
crates/codeoid-protocol/src/client.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   67.96%   68.24%   +0.28%     
==========================================
  Files          32       32              
  Lines        7834     7976     +142     
==========================================
+ Hits         5324     5443     +119     
- Misses       2510     2533      +23     

Flag	Coverage Δ
tui	68.24% <83.91%> (+0.28%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
crates/codeoid-client/src/connection.rs	23.34% <100.00%> (+1.80%)	⬆️
crates/codeoid-tui/src/commands/mod.rs	95.85% <100.00%> (+0.20%)	⬆️
crates/codeoid-protocol/src/client.rs	29.62% <0.00%> (-1.14%)	⬇️
crates/codeoid-tui/src/app.rs	38.84% <81.35%> (+3.33%)	⬆️