Add --single-dependency flag to jhack sync

[PietroPasotti]

Summary

Adds a --single-dependency / -d flag to jhack sync that watches a local Python package directory and syncs changes into the charm's remote venv site-packages.

Closes #238

Usage

jhack sync opensearch/0 --single-dependency=./opensearch_single_kernel

Instead of the current workaround:

jhack sync --source ./opensearch_single_kernel/ \
  --remote-root /var/lib/juju/agents/unit-opensearch-0/charm/venv/lib/python3.12/site-packages/opensearch_single_kernel/

What it does

1. Auto-detects the package name from pyproject.toml ([project].name or [tool.poetry].name) or setup.cfg ([metadata].name), with PEP 503 normalization (lowercase, hyphens → underscores). Falls back to the directory name.

2. Discovers the remote Python version by listing <charm_venv>/lib/ on the target unit via juju ssh, so the user doesn't need to know or specify python3.12 vs python3.10 etc.

3. Constructs the full remote path automatically:
   <venv>/lib/<python3.X>/site-packages/<pkg_name>/

4. Validates flag incompatibilities — errors out early if combined with --venv, --remote-root, or --source (which are all overridden by --single-dependency).

Implementation details

• New helpers: _resolve_package_name() and _discover_remote_python_version() in jhack/utils/sync.py
• New local_root_override parameter on push_to_remote_juju_unit() to correctly map files outside CWD to remote paths (fully backward-compatible — defaults to os.getcwd() when unset)
• All existing tests pass (208 passed)