Add unit tests for contribarena.engine.lifecycle pure helper functions

[northline-lab]

Summary

Add 44 unit tests for previously untested pure helper functions in contribarena.engine.lifecycle.

Functions now tested

Helper	Purpose
_has_patch_diff	Detects diff --git markers in a patch string
_patch_paths	Extracts sorted, deduplicated file paths from patch headers
_suspicious_patch_paths	Flags pycache/pyc/pyo/DS_Store/egg-info/pytest_cache in a patch
_changed_line_count	Counts +/- lines while ignoring ---/+++ headers
_branch_name	Generates slugified branch names from task title and run id
render_pr_description	Renders a PullRequestDraft as a markdown PR description
live_action_log_entries	Produces audit log entries for dry-run PR preparation
_postmortem_lesson	Selects lesson text from terminal/quality/CI state
render_postmortem	Renders the full postmortem markdown report

Risk

Low — this is a tests-only change that adds a single new file without touching any production code.

Verification

pytest tests/unit/test_lifecycle_helpers.py -v   # 44 passed in 0.26s
ruff check tests/unit/test_lifecycle_helpers.py  # All checks passed

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Prepared by ContribArena in owned-live mode.