Use Rich trees for rhiza tree output

[Copilot]

The tree command was hand-rolling ASCII tree rendering (├──, └──, │ ) via custom _build_tree / _render_tree helpers instead of delegating to Rich's Tree component.

Changes

• src/rhiza/commands/tree.py: Deleted _build_tree (nested dict) and _render_tree (ASCII line renderer). Replaced with _build_rich_tree which constructs a rich.tree.Tree directly, and uses rich.console.Console for output.
• tests/test_commands/test_tree.py: Replaced TestBuildTree / TestRenderTree with TestBuildRichTree targeting the new helper; integration tests unchanged.
• pyproject.toml: Added rich as an explicit dependency (was previously only a transitive dep via typer).

# Before
tree = _build_tree(lock.files)       # → nested dict
lines = _render_tree(tree)           # → list[str] with manual connectors
print(".")
for line in lines:
    print(line)

# After
rich_tree = _build_rich_tree(lock.files)   # → rich.tree.Tree
console = Console()
console.print(rich_tree)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Use rich trees</issue_title>
<issue_description>Do not try to implement your own tree treatment with display etc. Use them off the shelf</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Use rich trees #435

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