PR #27371 introduced three new identity-mapping config keys
(pinPeerName, userPeerAliases, runtimePeerPrefix), but the README's
'Full Configuration Reference' didn't mention them. Operators had
to read the source to understand the resolver, leading to predictable
support questions ("why is my user split across two peers?", "what
does pinPeerName actually pin?").
Add a new 'Identity Mapping' subsection that covers:
* The four config keys (pinUserPeer + alias, userPeerAliases,
runtimePeerPrefix) with concrete examples.
* The 7-step resolver ladder so operators can predict which peer a
given runtime ID will land on.
* Why there's no symmetric pinAiPeer (the AI peer is already pinned
by construction; the asymmetry is intentional).
* Host vs root semantics (host-level replaces root for maps, wipes
with empty value).
* The three deployment shapes ('hermes honcho setup' uses these same
shape names) with one-line guidance per shape.
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