feat: Pheby platform adapter/plugin for Hermes (protocol v1)

Third-party Hermes platform plugin serving the Pheby WebSocket+HTTPS
protocol for the native Android client, behind Caddy:

- Platform adapter (BasePlatformAdapter subclass) registered via the
  documented plugin system (plugins.platforms pattern, kind: platform)
- Multiple conversations mapped to Hermes sessions (authoritative state)
- Streaming chat via GatewayStreamConsumer edit path (message.delta)
- Structured tool events (never fake tool text in chat), incl.
  post_tool_call hook relay with Hermes tool_call_ids
- Native approval + clarification round-trips via tools.approval /
  tools.clarify_gateway primitives
- Attachment delivery: adapter-owned copies, opaque IDs, persistent
  metadata, 7-day retention + safe cleanup, authenticated HTTPS download
- Model + reasoning-effort query/change via Hermes picker data and
  session/global overrides
- Shared-secret auth (constant-time, lockout), size limits, path-
  traversal-proof attachment resolution, minimal health endpoint
- Protocol v1 spec with JSON examples (docs/PROTOCOL.md)
- Install/config/Caddy/security docs + discovered Hermes limitations
- 37 passing tests (auth, conversations, protocol, attachments incl.
  expiry/traversal, tool events, approvals, clarifications, cancel,
  reconnect re-sync, live WS smoke tests) — gateway-free fakes

No Hermes core modifications required.

.env.example

@@ -0,0 +1,12 @@
+# Pheby plugin environment template — copy real values into ~/.hermes/.env
+# NEVER commit the real .env.
+
+# Required: shared credential for every WS connection / attachment download
+PHEBY_SECRET=change-me-openssl-rand-hex-32
+
+# Optional
+#PHEBY_BIND_HOST=127.0.0.1
+#PHEBY_PORT=8620
+#PHEBY_DEBUG=false
+#PHEBY_LOG_CHAT_CONTENT=false
+#PHEBY_HOME_CHANNEL=<conversation-id-for-cron-delivery>