Read Codebase Context

Choose tools by purpose; current source and tests establish facts.

Refresh an impact map

When the active change's tasks.md (or a legacy plan) has an ## Impact map, first check that it has Verified at: <full commit SHA>. Verify that commit exists and is an ancestor of HEAD with git cat-file -e <sha>^{commit} and git merge-base --is-ancestor <sha> HEAD. If the SHA is missing, unavailable, or not an ancestor (for example, after a rebase), discard the map for navigation and trace existing source below. When no commit existed at map creation, re-trace existing source; if the planned source does not exist yet, use the source-less map rule in step 5 instead of tracing a nonexistent flow.