Add custom trace folder projects

[kargarisaac]

Summary

• add custom project source type via lerim project add <path> --type custom
• scan already-clean canonical JSONL folders directly as agent_type=custom, without platform adapters or compaction
• document the custom-agent flow, pasteable cleaner prompt, and per-agent architecture Mermaid diagrams
• bump package version and changelog for v0.2.1

Validation

• uv run ruff check src tests
• tests/run_tests.sh unit -> 1457 passed
• LERIM_INTEGRATION=1 uv run pytest tests/integration/custom_traces/test_cases.py -q -> 1 passed
• uv run --extra docs mkdocs build --strict --site-dir /tmp/lerim-docs-v021
• uv build -> built dist/lerim-0.2.1.tar.gz and dist/lerim-0.2.1-py3-none-any.whl