Each row of an agent's linked accounts, except its own `main` matrix
account, gets a delete action. swarm-controller serves DELETE beside each
PUT (matrix-accounts/{account}, forge-accounts/{label}, github-account),
answers 404 for an account the store does not hold, refuses `main`, and
removes every version through `delete_all_versions`.
The matrix confirmation has a revoke checkbox, off by default: the
controller logs the stored token out at its homeserver first, and keeps
the account when that fails or no homeserver is stored.
The controller's policy gains `delete` on each agent's
`metadata/.../matrix/+`, `forge/+` and `github-token`, pinned in
bao-grants.nix.
Refs #4855
bbf931207f