fix(pty): keep registration out of purged temp directories

[jlongster]

Summary

• Replace OPENCODE_PTY_RUNTIME_DIR with explicit flags. Every command (daemon, play, status, list, watch, stop) requires --name NAME and accepts an optional --runtime-dir DIR; the runtime directory is DIR/NAME. Names must be one path component of letters, digits, ., _, or -.
• DIR defaults to OpenCode's state directory, ${XDG_STATE_HOME:-~/.local/state}/opencode/pty, instead of $XDG_RUNTIME_DIR or the temp directory. macOS dirhelper deletes regular files in $TMPDIR whose creation and access times are over three days old, even while the daemon runs; this removed service.json and broke restart handoff discovery. The held service.lock survived because dirhelper skips open files.
• Keep the socket under /tmp/opencode-pty-<uid>/ so its path always fits sun_path; macOS temp cleaners skip sockets.
• On exit, remove the socket only if it is still this daemon's inode (matching remove_if_current for the registration), then remove the empty runtime directory.
• At startup, sweep sibling runtime directories in DIR that are over ten minutes old, contain only registration files, and show daemon evidence: a lock that can be acquired, or, without a lock file, a valid registration whose PID is gone. Empty directories and directories with foreign or unparseable files are kept.
• Document the flags, locations, and cleanup in README and SPEC.

Validation

• cargo fmt --check, cargo clippy --all-targets --all-features -- -D warnings, and cargo test pass on macOS.
• New tests cover default root resolution, name validation, sweep decisions (abandoned, young, foreign files, locked, live/dead PID, leftover socket, empty or foreign-registration directories kept), and that shutdown keeps a replaced socket. Existing daemon and playground tests use isolated --runtime-dir roots and assert the runtime directory is removed on exit.
• Manual: play --name demo --runtime-dir DIR was found by status/list with the same flags, swept a backdated abandoned sibling, and left DIR empty after quit; a missing or invalid --name is rejected.

Rollout

Breaking CLI change: OpenCode must spawn daemon --runtime-dir <root> --name <id> instead of setting OPENCODE_PTY_RUNTIME_DIR, together with the release bump. OPENCODE_PTY_RUNTIME_DIR is removed entirely. Existing handoffs carry their directory, so daemons still running from the legacy temp directory remain adoptable: OpenCode moves their service.lock and service.json into <root>/<id> by rename (the inode, and so the old daemon's flock, is preserved) and uses that directory from then on. If any step fails, OpenCode abandons the old daemon and starts a fresh one in a new directory; it never respawns into a legacy temp directory.