feat(cli): add attach-only mode - #123

Why

pty attach currently prompts to restart retained dead-session metadata. A
TTY-backed relay can remain at that prompt after its target dies, then consume
later input intended for the target as permission to execute the stored command.

Closes #122.

What

Add pty attach --no-restart <ref> as an explicit attach-only policy:

• running sessions attach normally;
• missing, exited, and vanished sessions exit nonzero without prompting;
• an attached client exits with the daemon and cannot start a second
  incarnation.

The existing interactive prompt and --auto-restart behavior remain unchanged.
Focused help, generated completion metadata, README usage, and the changelog
document the new flag.

How

The CLI now models attach restart behavior as a three-state policy:
prompt | always | never. The never branch refuses before entering the
dead-session presentation/restart path, so it does not display or execute
retained launch metadata. Contradictory --no-restart --auto-restart input is
rejected.

Rationale

This keeps the safety policy at the CLI boundary where the unsafe prompt exists,
while leaving the lower-level attach client and the existing interactive
contract intact.

Tests

• npm run typecheck
• npm run build
• npx vitest run tests/attach-no-restart.test.ts tests/help.test.ts tests/completions.test.ts tests/nesting-prevention.test.ts (57 passed)
• nix build .#pty --no-link
• Full local suite: 1,430 passed, 20 skipped, 4 environment-dependent failures
  in untouched shell/screenshot tests (zsh absent from the local Nix dev shell;
  local Vim resolves to Neovim and Vim/nano terminal-query screenshots time out).

The new regression suite covers missing, exited, vanished, running-then-exited,
delayed-input/no-second-incarnation, default interactive compatibility, and
--auto-restart compatibility.

Companion eval coverage: compoundingtech/evals#42.