fix(clean): reduce Python bytecode output spam

[sebastianbreguel]

Summary

Fixes the output spam and performance issue when mo clean encounters many __pycache__ directories.

Users reported 3000+ lines of Python bytecode cache · X, 1 dirs, 0 B output and 10-15 minute runtimes (#633). The root causes were: (1) empty __pycache__ dirs left from previous cleanups still being processed, and (2) the scan traversing conda/pip site-packages trees with thousands of package __pycache__ dirs.

Changes

1. Skip empty __pycache__ dirs at scan time

scan_project_cache_root now checks whether each __pycache__ directory contains .pyc or .pyo files before including it in the results. Empty dirs (left over from previous cleanups or created by tooling) are silently skipped.

2. Prune conda and site-packages from scan tree

Added miniconda3, anaconda3, miniforge3, mambaforge, and site-packages to the find prune list. These directories contain __pycache__ dirs from installed Python packages — cleaning them would break installed packages, and traversing them was the main cause of 10-15 minute scan times.

Visual preview

Before (from #633):

  ✓ Python bytecode cache · dev, 1 dirs, 0 B
  ✓ Python bytecode cache · dev, 1 dirs, 0 B
  ✓ Python bytecode cache · dev, 1 dirs, 0 B
  ... (3000+ lines, 10-15 minutes)

After:

  ✓ Python bytecode cache · my-project, 4 dirs, 1.2 MB
  ✓ Python bytecode cache · another-app, 2 dirs, 256 KB

Only projects with actual bytecode files produce output.

What changed

File	Change
lib/clean/caches.sh	Filter empty __pycache__ in scan_project_cache_root; add conda/site-packages to prune list
tests/clean_system_caches.bats	2 new tests: empty pycache skipping, conda/site-packages pruning

Test plan

• bats tests/clean_system_caches.bats — all 18 tests pass (2 new + 16 existing)
• Empty __pycache__ dirs excluded from scan results
• conda/site-packages paths pruned from scan tree
• Existing pycache grouping, dry-run export, and whitelist tests unaffected
• No regressions in project cache detection (configured roots, auto-detect, GOPATH-style)

Closes #633