docs(trust): document attested_peering; drop dead attested_peers accessor

Description

The pitch.

Follow-up polish to #149 (merged, thank you!). Two small, non-blocking items that
landed just after the merge:

### 1. `docs/layers/trust.md` — document the plugin
Adds an `## attested_peering` section alongside the other trust plugins
(`score_average`, `agent_receipts`, `parc`, `aae_permit_gate`, `bonded_trust`),
matching the house style: what it does, the three-question handshake, the
determinism story, a runnable quickstart, and the `attested_sybil_quarantined`
scenario/validator link. Every other plugin in the layer has an entry; this fills
the gap.

### 2. Remove a dead accessor
`AttestedPeeringTrust.attested_peers` (a `frozenset` property) had **zero**
references across the repo — `is_attested(agent)` covers the same query and is the
only accessor the tests/validators use. Trims the largest plugin in the batch by
10 lines with no behavior change.

### Verification
```
make ci-local   # ruff + format + pyright + pytest: 1037 passed
```

No source-behavior change; the plugin, scenario, validators, and all 145
plugin/validator tests are untouched.