docs: peering trust terminology — names for the four concepts (#134)

Gives stable names to what issue #134 showed gets conflated: Trusted peer
(invite-verified, operator decision), Discovered peer (learned from a
Trusted peer's advertisement, hard-capped at Observer — TRUST IS NOT
TRANSITIVE), Routing hint (what a Discovered peer actually contributes:
reachability, not trust), and Peer advertisement (the mechanism itself,
a feature not a leak).

Records the two rules that make the model sound (trust requires a
traceable operator decision; discovery is transitive, trust is not), why
advertisement exists (one invite makes a node reachable to the trusted
set without granting anything), and the deferred open questions: the
'don't advertise my peers' privacy toggle and UI tier vocabulary.

docs/README.md

@@ -54,6 +54,7 @@ step-by-step guides, and some predate the current implementation.
 - [Dual Ecash](dual-ecash-design.md)
 - [Hardware Signer](hardware-signer-design.md)
 - [Manifest Hooks](manifest-hooks-design.md)
+- [Peering & Federation Trust](peering-trust-model.md) — naming/semantics of trust levels vs discovery (#134)
 - [kdump + rasdaemon Troubleshooting](kdump-rasdaemon-design.md) — post-mortem and hardware-error capture on nodes (#144)
 - [Meshroller Integration](meshroller-integration-design.md)
 - [Nostr Git Source Hosting](nostr-git-source-hosting.md)