Add cmux support to agent-cli dev

[basnijholt]

Summary

Adds cmux (Ghostty-based macOS terminal for AI coding agents, controlled via a Unix socket through its bundled CLI) as a supported terminal for agent-cli dev.

The repo↔cmux mapping: one workspace per repository name, one tab per worktree.

• First launch for a repo: cmux workspace create --name <repo> --cwd <worktree> --command <agent-cmd> creates the workspace with the agent running in its first tab.
• Subsequent launches: cmux new-surface --workspace <ref> opens a tab in the existing workspace (matched by title via cmux workspace list --json), rename-tab names it after the branch, and cmux send types cd <worktree> && <agent-cmd> plus Enter. New surfaces don't accept cwd/command flags, so typing into the shell is the supported route; the terminal buffers the input until the shell is ready (verified live).

Changes

• New Cmux terminal adapter (agent_cli/dev/terminals/cmux.py), detected via the CMUX_WORKSPACE_ID/CMUX_SURFACE_ID env vars that cmux auto-sets in its terminals
• Registered in the terminal registry after tmux/zellij, so multiplexers running inside a cmux tab still win detection
• Detection-only, like other GUI terminals — no --multiplexer involvement
• Success summary shows cmux Workspace: <repo>; the tmux Attach: hint is no longer printed for non-tmux handles
• Tests with evidence-based docstrings (cmux CLI help contracts + live verification against cmux 0.64.14), docs table

Testing

• pytest: 1411 passed, 4 skipped
• pre-commit run --all-files: passes (jscpd fails locally on a clean tree due to a missing platform binary, unrelated)
• Live-tested against a running cmux app: first dev new in a scratch repo created the repo-named workspace with a branch-named tab running Claude in the worktree; second dev new reused the workspace and added a second tab whose agent started in the second worktree's directory

[greptile-apps]

Greptile Summary

This PR adds cmux (a Ghostty-based macOS terminal) as a supported terminal adapter for agent-cli dev. The design maps one cmux workspace per repository name and one tab per worktree launch, with full error recovery (e.g., closing idle tabs when send fails).

• New Cmux terminal adapter in agent_cli/dev/terminals/cmux.py with workspace create/reuse logic, deterministic per-workspace colors, and robust error handling via a shared _run helper.
• _launch_in_cmux added to launch.py (mirroring the existing _launch_in_tmux pattern); cmux detection registers after tmux/zellij so inner multiplexers win.
• Summary output in cli.py is narrowed with explicit terminal_name guards so the tmux attach hint is not printed for non-tmux handles.

Confidence Score: 5/5

Safe to merge — the adapter is well-isolated, all three previously-raised blocking issues have been addressed, and the test suite passes with 1411 tests.

All previously-raised issues (silent send failure masking a dead tab, indistinguishable CLI failure from workspace-not-found, and workspace-name divergence in open_new_tab) are resolved in this iteration. Error recovery is explicit: failed send closes the idle surface and returns None. The workspace listing abort-on-nil guard prevents spurious create attempts. The open_new_tab path now calls get_main_repo_root matching _launch_in_cmux. No new defects found in the changed paths.

No files require special attention.

Important Files Changed

Filename	Overview
agent_cli/dev/terminals/cmux.py	New terminal adapter; workspace create/reuse logic, per-workspace color hashing, and proper cleanup when send fails. All previously-noted blocking issues have been addressed.
agent_cli/dev/launch.py	Adds _launch_in_cmux following the same pattern as _launch_in_tmux; correctly short-circuits _launch_in_terminal before the generic open_new_tab path.
agent_cli/dev/cli.py	Summary panel now guards session_name display with explicit terminal_name checks for tmux and cmux; the tmux attach hint is no longer shown for non-tmux handles.
agent_cli/dev/terminals/registry.py	Cmux registered after Zellij, before GUI terminals — correct priority so tmux/zellij inside a cmux tab still win detection.
tests/dev/test_terminals.py	Comprehensive tests covering detection, workspace create/reuse, color determinism, send-failure cleanup, and priority ordering; evidence-backed docstrings cite live verification.
tests/dev/test_launch.py	New test verifies that launch_agent with a detected cmux terminal uses open_in_workspace with the repo-root name, confirming the workspace naming invariant through the launch path.
docs/commands/dev.md	Documentation table updated to include cmux with its detection env var and behavior summary.

Sequence Diagram

sequenceDiagram
    participant LC as launch_agent()
    participant LT as _launch_in_terminal()
    participant LC2 as _launch_in_cmux()
    participant CX as Cmux adapter
    participant CLI as cmux CLI

    LC->>LT: "terminal.name == "cmux""
    LT->>LC2: path, agent, terminal, full_cmd, tab_name, repo_root
    LC2->>CX: "open_in_workspace(path, cmd, tab_name, workspace_name=repo_root.name)"
    CX->>CLI: workspace list --json
    CLI-->>CX: "{"workspaces": [...]}"

    alt workspace already exists
        CX->>CLI: new-surface --workspace workspace:N
        CLI-->>CX: OK surface:M pane:P workspace:N
        CX->>CLI: rename-tab --workspace workspace:N --surface surface:M --title tab_name
        CX->>CLI: "send --workspace workspace:N --surface surface:M -- "cd /path && cmd\n""
        alt send succeeds
            CLI-->>CX: OK surface:M workspace:N
            CX-->>LC2: "TerminalHandle(handle=surface:M, session_name=workspace_name)"
        else send fails
            CLI-->>CX: CalledProcessError
            CX->>CLI: close-surface --workspace workspace:N --surface surface:M
            CX-->>LC2: None
        end
    else workspace not found
        CX->>CLI: workspace create --name repo --cwd /path --command cmd
        CLI-->>CX: OK workspace:N
        CX->>CLI: workspace-action --action set-color --workspace workspace:N --color Color
        CX->>CLI: rename-tab --workspace workspace:N --title tab_name
        CX-->>LC2: "TerminalHandle(handle=workspace:N, session_name=workspace_name)"
    end

    LC2-->>LT: TerminalHandle or None
    LT-->>LC: (success, handle)

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_{Reviews (4): Last reviewed commit: "Fix cmux tests on Windows by computing e..." | Re-trigger Greptile}