What it means
A projection declared with mode: region owns the text between <!-- majordomus:begin <policy hash> <content hash> --> and <!-- majordomus:end --> and nothing else. Everything outside those two lines belongs to the repository, is never read as input, and is never rewritten. update splices the rendered body into the region; doctor and watch compare the region body with the content hash in its own marker, never the file.
How it works
lib/common.sh carries the primitives: mj_region_extract returns the current region body and distinguishes no begin marker from malformed markers; mj_region_splice rebuilds the whole file around a new body; mj_projection_mode defaults an unset mode to file, so every existing policy keeps its behaviour. lib/update.sh renders the provider template into the region and writes the policy hash and the region body's hash into the begin marker. An unknown mode is a policy error and exits 10 rather than guessing. lib/doctor.sh and lib/watch.sh read the mode from the policy and compare the region body with its marker; the budget, link and count checks measure the region and report the host document's length as information, because the host is not the tool's to shrink.
How to see it
majordomus update # writes the region into CLAUDE.md
echo "a paragraph of my own" >> CLAUDE.md
majordomus doctor # ok — the edit is outside the region
sed -i '' 's/one active task/two active tasks/' CLAUDE.md
majordomus doctor
# FAIL projection CLAUDE.md — content differs from its own stamp (hand-edited?)What it does not cover
The tool does not create the markers for you in a file it did not write. If the begin marker is missing, update appends the region at the end of the file; where in the document the region belongs is an editorial decision. Markers that are present but malformed — a begin without an end, or an end before its begin — are refused rather than repaired.
Why it exists
A repository that already has a hand-authored CLAUDE.md had to choose between keeping its own governance root and adopting the tool. mode: region removes the choice: the authored text stays authoritative and the generated supervisory section sits inside it, stamped like any other projection.