feat(project-coordinator): offer to keep a history of a project

A project folder accumulates work with no way to see what changed or undo a
wrong turn. The coordinator now offers to keep one, once, in plain words, and
initializes it only after an explicit yes — the mechanism is git in the sandbox
but the jargon stays out of the conversation, since the person being offered
this is not necessarily someone who knows what a commit is. That first yes is
standing consent to snapshot at later milestones, so the agent does not re-ask
each time.

Recorded in the project's SKALD.md so a future session knows the history exists
rather than proposing it again, and documented in docs/projects.md, which is
what the assistant reads to explain the feature to a user.

docs/projects.md

@@ -73,3 +73,4 @@ Access changes apply immediately — no need for the other person to log out.
 - A private project is simply a project with one member (you). Share it later whenever you want.
 - Renaming a project does not move its folder, so links and the assistant's context keep working.
 - Deleting a project removes its folder for everyone — there is no undo.
+- The assistant may offer to **keep a history** of the project: a trail of snapshots you can look back on, or return to if something goes wrong. If you accept, it notes that in the project's `SKALD.md` and saves a new snapshot whenever the project reaches a meaningful milestone. The history lives inside the project folder on the server (it is powered by git, but you never need to touch it).