Improve gitwise sync --pull UX for diverged branches

[drzioner]

Problem

gitwise sync --pull currently fails with a generic message when local and remote have diverged.
The command intentionally uses git pull --ff-only for safety, but users do not get enough actionable guidance.

Current message:

error: pull failed: local and remote have diverged (use merge or rebase)

Why this is confusing

• Users often expect --pull to "just sync".
• The command is conservative by design (FF-only), but that safety model is not obvious in the error path.

Proposed improvements

1. Improve diverged-pull hints with explicit next steps:
   • gitwise sync --dry-run --json to inspect ahead/behind
   • git pull --rebase (recommended)
   • git pull --no-rebase (merge flow)
   • gitwise sync --push after reconciliation
2. Document FF-only default and divergence handling in command reference under gitwise sync.
3. Optional follow-up: support strategy flag:
   • gitwise sync --pull --strategy ff-only|rebase|merge
   • keep default ff-only for backward-compatible safety.

Acceptance criteria

• Diverged pull error shows concrete commands in EN/ES.
• JSON mode includes structured hint for sync_pull_diverged with strategy guidance.
• Docs clearly explain FF-only default and divergence resolution.

Notes

This should be UX-only unless a separate proposal explicitly changes default safety semantics.