feat: detect uv Python projects

[QueryPlanner]

What

Add uv project detection using uv.lock and pyproject.toml.

Why

Python detection currently requires a .py file at the project root. uv projects with source files in a subdirectory are missed, leaving root-level caches undiscovered.

How

• Remove an existing redundant reference in CLI formatting that Rust 1.98 Clippy rejects. CLI output remains unchanged.
• Recognize both marker filenames as the existing Python project type in scanning and direct cleanup.
• Require regular files for the new direct-cleanup markers so same-named directories or symlinks cannot trigger cleanup.
• Reuse the existing Python cache directories. Preserve .venv, following the policy established in dont remove .venv for python #182.
• Recognize any pyproject.toml, including files with [tool.uv], without adding a TOML parser or dependencies.
• Document that cleanup applies to cache directories at the detected project root. Recursive cache cleanup and custom uv environments are outside this change.

Tests

• cargo fmt --all -- --check
• cargo clippy --tests --locked --offline -- -D warnings
• cargo clippy -p kondo-lib --tests --locked --offline -- -D warnings
• cargo test --all --locked --offline: 1 unit test and 4 integration tests pass.
• cargo build --locked --offline
• Regression tests cover both markers, generic and uv pyproject files, multiple markers, legacy Python scanning, cache sizes, both cleanup APIs, directory-marker rejection, and preservation of source files, markers, and a non-empty .venv.
• The two feature regression tests fail against an isolated copy of unchanged upstream.
• CLI dry-run detects both marker-only fixture roots, excludes .venv, ignores directory/symlink markers, and preserves all fixture contents.
• LLVM instrumentation covers all 6 added detection executable lines. Whole-library coverage is 233/370 lines; this is not a claim of full repository coverage.

Local checks initially ran on macOS arm64 with Rust 1.96.0. After CI exposed a newer Clippy lint, validation was repeated with Rust 1.98.0, matching CI. The original failure was reproduced locally before the patch, and CLI dry-run output was compared before and after it. The workspace emits an existing future-compatibility warning for the transitive block dependency; all commands above pass.

Related Issues

Closes #188.

The uv project layout is documented at https://docs.astral.sh/uv/concepts/projects/layout/.