fix: harden diff ref parsing in git worktree flow

[bhattaraiprayag]

Scope

This PR contains one focused bug fix for branch-diff ref handling, plus regression tests for that same behavior.

Why

axon diff passed user-provided refs directly to git worktree add, so refs starting with - could be parsed as git options.

What changed

• Added _validate_ref(repo_path, ref) in src/axon/core/diff.py:
  • rejects refs that start with -
  • validates refs with git rev-parse --verify --quiet --end-of-options <ref>^{commit}
• diff_branches() now validates base_ref and optional current_ref before graph build/worktree operations.
• _build_graph_for_ref() now calls:
  • git worktree add <path> -- <ref>
    so user refs are always passed after end-of-options.
• Added regression tests in tests/core/test_diff.py for:
  • accepted refs (main, feature/x, HEAD~1)
  • rejected dash-prefixed refs
  • rejected literal $(id) refs (treated as plain ref text)
  • pre-build validation in diff_branches
  • -- separator presence in worktree command args

Validation

• uv run pytest -q tests/core/test_diff.py -> 23 passed
• uv run pytest -q -> 591 passed
• uv run ruff check src/axon/core/diff.py tests/core/test_diff.py -> pass
• uv run ruff format --check src/axon/core/diff.py tests/core/test_diff.py -> pass
• Runtime behavior checks:
  • axon diff -- '--detach' -> rejected
  • axon diff -- '$(id)' -> rejected as invalid ref
  • axon diff 'HEAD~1..HEAD' -> succeeds for valid refs

Closes #35
Closes #38

Checklist

• This PR is linked to a maintainer-approved issue (or is a trivial typo/link fix under 5 lines)
• I have read CONTRIBUTING.md
• One logical change only — no bundled/unrelated work
• Tests added or updated for this change
• pytest passes locally
• ruff check src/ tests/ passes
• I can explain every line of this PR when asked
• No generated project-config files (CLAUDE.md, .cursorrules, etc.)