Remap relative changed_files to absolute paths in analyze_changes

The affected-flows lookup and the no-ranges node fallback match
changed_files against nodes.file_path, which stores absolute normalized
paths. CLI callers pass repo-relative diff paths, so the exact IN (...)
match found no nodes and detect-changes reported 0 affected flows while
the MCP tool reported hundreds on identical input.

Remap changed_files the same way analyze_changes already remaps
changed_ranges keys. Already-absolute inputs pass through pathlib
joining unchanged, so the MCP path is unaffected.

Fixes #848