Migrate AutoRespondCoordinator CLI string selection to backends

[dgershman]

Follow-up to #410 / ADR 0005.

Today AutoRespondCoordinator selects the CLI binary name with inline ternaries:

provider == .gitlab ? \"glab\" : \"gh\"

in six places: Sources/Crow/App/AutoRespondCoordinator.swift:127,133,153,170,197,206.

With the new TaskBackend / CodeBackend abstraction in place, this can move
behind a backend method (e.g. backend.cliName or a richer prompt-vending API).
The prompt body still legitimately branches on provider for prose differences;
only the CLI token selection should move.

Acceptance

• All six call sites at the lines above use the backend abstraction instead of
  provider == .gitlab ? "glab" : "gh".
• No behavior change in the resulting prompts.

References:

• ADR 0005: docs/adr/0005-task-and-code-backend-protocols.md
• PR Extract TaskBackend + CodeBackend protocols (#410) #411