feat: add python3/pytest output filter — compress test results and script output

Problem

python3 and virtual-env Python commands produce verbose output that wastes tokens:

• python3 -m pytest — full test output with per-test lines, warnings, tracebacks
• .venv/bin/python -m pytest — same, but absolute path bypasses rewrite
• /opt/homebrew/bin/python3 -c "..." — inline scripts with large stdout
• python3 << 'PYEOF' — heredoc scripts (related: #1243)

Evidence

rtk discover (last 30 days):

• python3 <<: 47 commands
• .venv/bin/python: 16 commands
• /opt/homebrew/bin/python3: 15 commands
• Total: 78 commands, currently unhandled

Suggested approach

1. Rewrite coverage: match python3, .venv/bin/python, absolute-path python binaries
2. pytest filter: compress per-test PASSED/FAILED lines, keep summary + first failure traceback
3. General python filter: truncate large stdout, keep first/last N lines

Environment

• rtk 0.30.0, macOS Darwin 24.6.0
• Claude Code PreToolUse hook