D272 — Writes carry an optional reason, stored as a Reason: commit trailer

Decision. Every git-wrapped write tool accepts an optional reason string. gitWrap adds it to the tool's schema and appends it to the commit as a Reason: <text> trailer after a blank line; the subject is unchanged. changes_since returns the distinct reasons per concept as reasons.

Why. History recorded what changed (concept_write: infra/dns) and never why, so a return-to-work digest was a list of IDs. A trailer keeps every consumer of subjects working and is parseable by git itself (%(trailers:key=Reason,valueonly)), with no custom body grammar. Injecting it centrally leaves the handlers untouched, which ignore unknown keys anyway. The reason is normalised (trimmed, whitespace collapsed, 500 bytes at a rune boundary) so one write cannot produce a multi-paragraph body.

Alternatives rejected. - A reason argument edited into each handler: dozens of edits, and a future tool would forget it. The registry test now catches that instead. - Putting the reason in the subject: breaks parsers of subjects and ops. - Recording it in the audit log: its field list is a leakage allow-list and free text may carry anything. - Appending it to log.md: that file stays the curated narrative.

Consequences. Tool.GitWrapped marks the tools that got the property; a tool that already declares reason keeps its own schema entry. gitx.CommitChanges has a Reason field (first trailer only) and the LogNameStatus format has a fifth NUL-separated field, which later readers of the log build on. changes_since keeps at most 5 reasons per concept.