GIT_TERMINAL_PROMPT=0 only suppresses git's own credential prompts. When
fetching over SSH, the authentication prompts (host key confirmation,
password, key passphrase) are emitted by `ssh` directly to the controlling
terminal, bypassing the piped stderr that we parse for progress. With the
progress bars drawn on that same terminal, the prompt is hidden behind them
and the fetch appears to hang indefinitely -- typically for users without a
configured SSH key.

Set GIT_SSH_COMMAND to append `-o BatchMode=yes` so ssh refuses to prompt
and fails fast with a clear error instead, mirroring the GIT_TERMINAL_PROMPT
treatment for git. Any user-provided GIT_SSH_COMMAND is preserved, and since
ssh honors the first value seen for an option, an explicit BatchMode set by
the user still takes precedence.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

…erations

Following review feedback that fully disabling input is too restrictive,
soften the SSH handling and make failures actionable instead of silent:

- Add `-o StrictHostKeyChecking=accept-new` so the common
first-connection
  host-key prompt no longer blocks (changed keys are still rejected).
This
  removes the most frequent prompt without compromising on key changes.
- Detect authentication failures in git's stderr and emit a targeted
help
  message (configure ssh-agent / a passphrase-less key, check repo
access)
  rather than just dumping the raw stderr.

Interactive credential prompts remain suppressed because they cannot be
shown safely while fetching in parallel behind progress bars.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Allow interactive auth with --no-progress instead of auto-accepting

Per review feedback, drop the auto-accept of unknown host keys and
instead
make interactive authentication possible by turning progress bars off.

Prompts from git and ssh are written directly to the controlling
terminal,
so they only collide with the progress bars visually; with the bars off
the
prompt is visible and answerable. Suppression of prompts is now
conditional:

- progress bars active, or no terminal at all (e.g. CI): suppress
prompts
  (GIT_TERMINAL_PROMPT=0 + ssh BatchMode=yes) so the command fails fast
  instead of hanging on input nobody can see or provide.
- interactive terminal with --no-progress: leave prompting untouched, so
  credentials and host keys can be entered normally.

The auth-failure help message now tells the user to re-run with
--no-progress to authenticate interactively (or to configure ssh-agent).

Adds Diagnostics::progress_active() to expose whether bars are
rendering.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Recommend --git-throttle 1 alongside --no-progress in auth hint

When authenticating interactively via --no-progress, concurrent fetches
can
still interleave their prompts on the shared terminal. Extend the
auth-failure
help to suggest `--git-throttle 1` so only one git operation prompts at
a time.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>