feat: inherit the focused session's working directory in boo ui

[kylecarbs]

In boo ui, C-a c now creates the new session in the focused session's working directory, like Ghostty's new windows, instead of inheriting the boo ui process's directory.

How it works

• The daemon resolves the live working directory of the session's child process and exposes it through a new cwd control command. Resolution reads the OS directly: /proc/<pid>/cwd on Linux, proc_pidinfo(PROC_PIDVNODEPATHINFO) on macOS.
• boo new gains a --cwd <dir> flag (validated, must be an existing directory). The child chdirs there before exec.
• The UI queries the focused session's cwd and passes it through to boo new --cwd. If the directory can't be resolved or used, it falls back to a plain new, so the keybind always works.

The directory comes from the live child process rather than OSC 7 shell integration: boo doesn't inject shell integration, and a process cwd is always a valid local path for chdir (OSC 7 can be empty, or describe a remote directory over ssh). See the decision log below.

boo new --cwd is also useful on its own, e.g. boo new build -d --cwd /srv/app -- make.

Decision log

• Mechanism: process cwd, not OSC 7. OSC 7 / term.getPwd() matches Ghostty and is portable, but boo never injects shell integration so it is usually empty, and under ssh it can report a remote path that is wrong for a local chdir. The child process cwd works without shell integration and is always a valid local path. OSC 7 could be added later as a fallback.
• Where it is resolved. Each session is its own daemon that owns the child pid, so the daemon answers a new cwd control command; the UI (a client) cannot read the child pid itself. cwd is its own command rather than a field on info so the extra syscall only happens on create, not on every ls/wait poll.
• macOS layout. proc_pidinfo only fills the buffer when its size exactly matches struct proc_vnodepathinfo, so the full <sys/proc_info.h> layout is reproduced. The size (2352) and the cdir path offset (152) were verified against the system headers and are locked with comptime asserts that run on every target.
• Validation and fallback. boo new --cwd resolves the path to an absolute, openable directory up front and fails with a clear error if it can't, so a bad value never produces a dead session. The UI retries without --cwd if the inherited create fails (e.g. the directory vanished).
• Naming unchanged. The default session name still derives from the creating process's directory; only the child's working directory is inherited.

Testing

• zig build (native Linux) and zig build -Dtarget=aarch64-macos (cross) both succeed; the macOS cross build exercises the proc_pidinfo path and the comptime layout asserts.
• zig build test (130/130) and zig build test-integration (76/76) pass; zig build test-all -Doptimize=ReleaseSafe passes (206/206). zig fmt --check is clean.
• New tests: cwd.ofPid against a child spawned in a temp dir; boo new --cwd starts the command in that directory; invalid --cwd exits non-zero; and a boo ui end-to-end test asserting C-a c lands the new session in the focused session's directory and not the UI's.

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