feat: add WSL/Linux terminal window launching support

[rexsilex]

Add platform-specific terminal backends for WSL (Windows Terminal) and native Linux (gnome-terminal, konsole, xterm) alongside the existing macOS backend. Extract terminal logic into backend modules while keeping the public API unchanged.

Key changes:

• Add platform-detect.ts for WSL/Linux environment detection
• Add terminal-backends/ with darwin, wsl, linux backends and factory
• Refactor terminal.ts to delegate to platform-specific backends
• Fix E2BIG on Linux by externalising large CLI args to temp files
• Fix race condition: write loom metadata before opening terminal tabs
• Fix empty error messages in Claude CLI error handling
• Update TerminalColorManager with WSL-aware messaging

WIP: Terminal tabs open and Claude launches, but behavior still being refined for full end-to-end WSL support.

[acreeger]

Thanks for this PR, @rexsilex! Great work on the terminal backend architecture — the strategy pattern with platform-specific backends is clean and well-tested. The E2BIG fix and race condition fix are solid additions too.

I ran a code review and found a few issues, which I've gone ahead and fixed in follow-up commits on this branch:

Issues Found & Fixed

Bug (High): Empty command produces shell syntax error on Linux

In linux.ts, when buildCommandSequence({}) returns a single space " ", the keep-alive construction produces " ; exec bash", which is a bash syntax error:

$ bash -lic ' ; exec bash'
bash: syntax error near unexpected token `;'

Fix: Trimmed the buildCommandSequence result and added an empty-string guard in both openSingle and openGnomeTerminalMultiTab, so an empty command produces just exec bash.

Bug (Medium): bash -c vs bash -lic inconsistency in gnome-terminal multi-tab

The single-tab path in execTerminal correctly uses bash -lic (login/interactive shell, loads profile scripts), but openGnomeTerminalMultiTab used bash -c. This means multi-tab launches on gnome-terminal wouldn't load .bashrc/.bash_profile, so tools like nvm, asdf, rvm etc. wouldn't be available.

Fix: Changed to bash -lic for consistency.

Security (Medium): Temp files written with default permissions

The externalizeIfNeeded function in claude.ts writes sensitive data (system prompts, MCP configs, agent configs) to temp files with default permissions. On Linux/WSL with a common umask of 022, these files would be world-readable (644). On shared multi-user systems (common in WSL enterprise setups), other local users could read these files.

Fix: Added mode: 0o700 for temp directories and mode: 0o600 for temp files.

Additional Notes (not fixed, low priority)

• escapeForBashC in wsl.ts is exported and tested but never called — consider removing or adding a comment about intended future use
• buildTerminalAppScript in darwin.ts duplicates command-building logic instead of using shared buildCommandSequence — potential refactor opportunity
• Duplicate detectITerm2 functions exist (async in terminal.ts vs sync in darwin backend)