feat(cli): add pty-relay completions <shell> (bash/fish/zsh)

[schickling-assistant]

Why

pty-relay ships no shell completions at all, while both sibling CLIs do:
pty gained pty completions <shell> in compoundingtech/pty#106, and
st has st completions. Downstream packaging (Nix, Home Manager) has
nothing to call.

What

Adds pty-relay completions <shell> for bash, fish and zsh.

All three scripts are emitted from ONE declarative spec of the command
tree in src/completions.ts, so they cannot drift apart. The spec is
derived from the actual switch dispatch in src/cli.ts — main,
dispatchServer, dispatchClient, dispatchLocal and the
CLIENT_PASSTHROUGH_COMMANDS set — not from the prose --help text.

Coverage: the 26 top-level commands (including the list/ls alias),
the server (11 verbs), client (signin/join + 9 passthroughs), local
(3 verbs) and clients (4 verbs) groups, their flags, the enum-valued
flags (init --backend, server rotate --role) and enum positionals
(server totp show|code, completions bash|fish|zsh).

How

• src/completions.ts is the generator. It follows pty's module shape
  (side-effect free, console/exit-code based, same FlagSpec/CommandSpec
  vocabulary) and borrows st's nested verbs because pty-relay — unlike
  pty — has namespaced groups. Verb names such as start/status/reset
  collide across groups, so the fish guards are scoped on both the verb and
  its group.
• src/cli.ts gains a case "completions". It sets process.exitCode
  rather than calling process.exit(), so a redirected script is fully
  flushed. completions is added to the list of commands excluded from the
  generic per-command --help short-circuit, so completions --help
  documents its own shell list.
• The module stays inside the --experimental-strip-types subset (type
  annotations and as const only), since pty-relay runs raw .ts.

Deliberately not dynamic

pty completes live session names off disk. pty-relay's peers live in the
encrypted secret store — the plaintext ~/.config/pty-relay/peers file is
optional and usually absent — so there is no cheap, reliable source for
host-label completion at completion time. Subcommands and flags only.

Verification

• test/completions.test.ts (new, 41 cases): dispatch and exit codes via
  spawnSync (importing cli.ts runs main()), the generated fish
  surface, a spec-covers-real-dispatch assertion listing every user-typeable
  case label, and bash -n / fish -n / zsh -n syntax checks that skip
  when the shell is absent. All pass.
• test/cli.test.ts still passes except two doctor cases that fail on
  main too, in any checkout without a linked @compoundingtech/pty.