secrets.md and credentials-reference.md answered the same question from two
pages. secrets.md's text now follows the reference's table and operating
notes as a 'Where each secret lives' section, unchanged apart from where it
sits. Every link and doc path that named secrets.md (docs, nix comments,
crate READMEs) points at credentials-reference.md; anchors are unchanged.
The overview keeps the store invariant, the mTLS exception and the
minter/reader/renewal contract. The table of every credential and the
operating notes (forge token checks, re-creating agents, revoking a queue
credential, store login retries) move unchanged to credentials-reference.md,
and the links that asked where a credential lives point there.
