Add docs/changes/ — a durable, in-repo map of every functional change and
the commit that records it, independent of git log. INDEX.md carries the
convention (each functional change gets a dated detail doc plus a ledger
row) and the full oldest-first ledger, regenerable losslessly from git.
Seed detail docs for the two changes just landed: the break-glass halt op
(c2ee21a) and the /felis migrate command (c1aa38b).
