prepare_canonical: unhelpful error when chgrp fails on files not owned by the user

[ligon]

Problem

When the canonical repository contains files not owned by the current
user (e.g. files created by the agent user or by root),
prepare_canonical fails silently or with a generic CommandError
because chgrp -R cannot change the group on those files.

The consequence for sucoder collaborate (which calls bootstrap →
prepare_canonical → ensure_clone) is:

1. prepare_canonical runs chgrp -R coder <canonical> — this
   partially fails (exit code 1) on files not owned by the user.
2. Because check=True, CommandError is raised with a generic
   "Command failed with exit code 1: chgrp -R coder …" message.
   The chgrp stderr (e.g. chgrp: changing group of '…': Operation not permitted) is logged but not surfaced clearly.
3. The MirrorError handler in the CLI prints the error and exits,
   but the user is left without a clear diagnosis or remediation path.

In practice this presents as an opaque failure (or, in some
configurations, a loop where prepare_canonical keeps retrying).

Expected behavior

When chgrp/chmod fails due to file ownership, prepare_canonical
should:

1. Detect the specific "Operation not permitted" failure.
2. List the offending files (or at least the first few).
3. Suggest a concrete fix, e.g.:

Cannot change group on 3 files not owned by you:
  src/data/cache.pkl  (owner: coder)
  .git/objects/pack/pack-abc123.idx  (owner: root)
  .git/objects/pack/pack-abc123.pack  (owner: root)

Fix with:
  sudo chown ligon:ligon <files…>
  # then re-run sucoder collaborate

Where to fix

• mirror.py:prepare_canonical (line ~203–205): catch CommandError
  from the chgrp -R call, parse stderr for "Operation not permitted"
  lines, extract the failing paths, and raise a MirrorError with
  actionable guidance.

Reproducer

# Create a file owned by root inside the canonical repo
sudo touch /path/to/canonical/root-owned-file

# Now try to collaborate
sucoder collaborate
# → opaque "Command failed" error

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