fix: harden lib-agent.sh PYTHONPATH against shadow module bypass

[fazxes]

Summary

• Replaces all 6 PYTHONPATH="$RECURSIVE_DIR/lib:$REPO_DIR" calls in .recursive/engine/lib-agent.sh with the safer _NS_LIB env-var + sys.path.insert(0, os.environ['_NS_LIB']) pattern
• Removes $REPO_DIR from every path assignment in lib-agent.sh — a shadow module placed in the project root could previously hijack imports
• Matches the pattern applied to daemon.sh in PR fix: harden daemon python3 -c against module shadowing and interpolation #190
• Call sites fixed: cleanup_old_logs, cleanup_healer_log, cleanup_orphan_branches, compact_handoffs, should_evaluate, run_evaluation

Verification

• bash -n .recursive/engine/lib-agent.sh passes (syntax clean)
• make check passes — 882 tests, ruff, mypy, shell syntax, ASCII checks all green
• No PYTHONPATH assignments remain in lib-agent.sh

Safety invariants

No safety invariants modified. This change touches the import mechanism for library helpers only; circuit breaker, cost cap, selective git add, zone compliance, prompt guard, lock file, and reset-to-origin logic are all unchanged.

Test plan

• bash -n .recursive/engine/lib-agent.sh - syntax check passes
• make check - full CI passes (882 tests)
• Grep confirms zero PYTHONPATH assignments remain in lib-agent.sh
• Grep confirms 6 _NS_LIB assignments and 6 sys.path.insert calls are present