Summary

• When a pack's context.md loses all its <!-- sync:fetch ... --> markers, the previously-written context.expanded.md was never deleted. Because the loader prefers context.expanded.md over context.md, sync + inject kept serving stale fetched content indefinitely.
• runMarkerExpansion now deletes the orphaned context.expanded.md for any pack that no longer has markers, so the loader falls back to the fresh context.md.
• Added a small removeStaleExpansion helper plus three unit tests covering: file is deleted, no-file-is-not-an-error, sibling context.md is left untouched.
• Documented the cleanup contract in CLAUDE.md and docs/content-authoring.md so pack authors can rely on it.

Repro (before the fix)

1. Add a <!-- sync:fetch ... --> marker to a pack's context.md and run sap-devs sync.
2. Remove the marker from context.md and run sap-devs sync again.
3. sap-devs inject still includes the fetched content because context.expanded.md is preferred and was never cleaned up.

Test plan

• go build ./...
• go vet ./...
• go test -c ./cmd/ (compiles; execution blocked locally by Windows Defender per the documented gotcha — CI runs them)
• CI green on ubuntu-latest
• Manually verified: deleted local stale context.expanded.md; next inject uses fresh context.md