M8 — Linux support

Context

clowder is macOS-centric today: the frontend is a SwiftUI app embedding libghostty (macos/), and packaging/signing/Homebrew are all macOS. The Rust core is cross-platform in principle:

• clowder-daemon (agent PTYs, control/hook sockets, worktrees) — Rust + portable-pty + unix sockets; likely portable to Linux with modest work.
• clowder CLI (clowder attach, clowder connect, spawn) — Rust, raw-mode terminal; likely portable.
• The macOS app is the hard part — there is no equivalent on Linux.

The M7 remote-daemon spec gestured at this (docs/superpowers/specs/2026-07-31-clowder-m7-remote-daemon-design.md, "the M8 Linux …" is listed as out-of-scope/future).

Needs scoping (its own brainstorm first)

This is a large, new direction that should be decomposed before committing to a plan. Open questions:

• Frontend on Linux: no SwiftUI/libghostty app. Options: (a) a pure-CLI / terminal-multiplexer client (run clowder attach <pane> panes inside tmux/zellij or a plain terminal), (b) a native Linux GUI embedding ghostty's GTK path, (c) rely on the M7 remote path — a Linux box runs clowder-daemon, the user drives it from the macOS app over TLS (already shipped in M7d, #53).
• Daemon portability: audit clowder-daemon/clowder-client/clowder-workspace for macOS-isms (paths, signals, PTY specifics); confirm the socket/runtime-dir logic ($XDG_RUNTIME_DIR already handled) and worktree provisioning work on Linux.
• CI/packaging: a Linux job (currently CI is runs-on: macos-15 only) + a Linux distribution story.

First step

Brainstorm → decompose into sub-projects (which pieces, what order), then spec → plan → SDD per slice. The cheapest early win may be (c): verify the Rust daemon builds+runs on Linux and is drivable from the existing macOS app via the M7d remote transport, deferring a native Linux frontend.

Read the M7 remote-daemon design in docs/superpowers/specs/2026-07-31-clowder-m7-remote-daemon-design.md, then audit clowder-daemon, clowder-client, and clowder-workspace for Linux portability. Verify whether the Rust daemon builds and runs on Linux and can use the existing M7d remote transport; done for this issue means decomposed sub-projects with an agreed order before slice-specific specs and plans.