Phase 2: open command + naming + outside-tmux attach

[dunnbot]

Summary

Implement the no-args entry path: `mux` in a project dir creates or attaches a session. Outside-tmux only in this phase (inside-tmux behavior comes with restore in Phase 3).

Depends on #1.

Approach

• `naming.rs` — port `~/.files/bin/mux:535-559`:
  • strip `$HOME/` and leading `.` from PWD
  • single segment → use as-is; multi-segment → `parent-base`
  • lowercase; replace `.` and `:` with `-`
• `tmux/mod.rs` — `Tmux` trait. Real impl wraps `std::process::Command`. Fake impl for tests.
• `commands/open.rs` — `mux` (no args) + `mux `:
  • register entry in TOML if absent
  • if session exists: attach / switch (no layout work yet — that's Phase 3)
  • if new: outside-tmux build chained `tmux new-session ; set-window-option automatic-rename off` invocation and `exec` it
• No pane restore, no command capture in this phase — just attach/create.

Acceptance Criteria

• `naming::tests` covers every case in the bash version (verify against PWDs in current `~/.files/tmux/sessions.json`)
• `cd /tmp/scratch && mux` from outside tmux creates session `scratch` and attaches
• `cd ~/Code/aniftyco/mux && mux` from outside tmux creates session `aniftyco-mux` and attaches
• `mux myname` overrides the derived name
• Re-running `mux` in the same dir attaches to the existing session

Files / Areas

• `src/naming.rs`
• `src/tmux/{mod,exec,fake}.rs`
• `src/commands/open.rs`
• `tests/naming.rs`, `tests/commands.rs` (open paths)

Notes

Inside-tmux behavior (tab-on-current, focus-primary-window) deferred to Phase 3 to keep this phase scoped to a single execution mode.