Fix tmux socket name length exceeding filesystem limits

[Zeko369]

Problem

When a project or branch name is long, the tmux socket path can exceed the unix-domain socket sun_path limit (104 bytes on macOS, 108 on Linux), so overmind start fails with File name too long and never starts tmux:

error connecting to /private/tmp/tmux-501/overmind-ud-2502-migrate-attribute-validation-rules-to-dashboard-cgszlXlXpWCM59i99NoRE (File name too long)

The socket name is the instance ID (overmind-<session>-<nanoid>), which tmux places under <tmux-tmpdir>/. With a 55-char session that path is 108 bytes — over the macOS limit.

Changes

1. Truncate the session to fit the actual socket-path budget. Rather than a fixed cap, the session portion of the instance ID is truncated to whatever room is left after accounting for the real tmux socket directory ($TMUX_TMPDIR or /tmp, plus tmux-<uid>, symlinks resolved) and the -<nanoid> suffix. This adapts to any uid length or custom TMUX_TMPDIR instead of assuming a particular environment; uniqueness is still guaranteed by the nanoid.

2. Clean up the command-center socket on a fatal exit. utils.Fatal calls os.Exit, which skips deferred cleanup — so when startup failed (e.g. the error above), .overmind.sock was left orphaned and the next run reported that overmind was "already running." Fatal now runs registered cleanup functions first, and the command center registers its socket teardown.

(These are two small, related startup-robustness fixes — happy to split them into separate PRs if you'd prefer.)

Testing

• Reproduced the original failure with a 55-char project name; with the fix the socket name is truncated and overmind start runs.
• Forced a tmux startup failure and confirmed .overmind.sock is removed on exit (previously leaked).
• gofmt / go vet clean; go build passes.