docs: README + env.example for search, PMs, webhook/SSO helpers

Wire table gains the search, private-message, webhook and SSO
rows; the library surface shows Search/SendMessage calls and a
server-side helper section (WebhookHandler, SSOLogin, secret
loaders); the CLI reference lists the four new commands including
their secret env vars. env.example documents the DISCOURSE_KEY
alias and the two commented-out secrets.

Part of Redmine 507 (Discourse Go client).

env.example

@@ -7,8 +7,17 @@ DISCOURSE_URL=https://community.turnsys.com
 
 # Discourse API key (Admin API > Keys, or a user API key). Creating
 # categories requires an admin-scoped key; a standard key 403s there.
+# DISCOURSE_KEY is accepted as an alias when DISCOURSE_API_KEY is unset.
 DISCOURSE_API_KEY=replace-me
 
 # The user the key acts as (must match the key's allowed username, or be
 # "system" for global keys).
 DISCOURSE_API_USERNAME=reachableceo
+
+# Webhook payload secret (Admin > Settings > API > web hook). Only the
+# `webhook verify` command and the server-side helpers need it.
+#DISCOURSE_WEBHOOK_SECRET=replace-me
+
+# Discourse Connect (SSO) shared secret (Admin > Settings > Login >
+# discourse connect secret). Only the SSO helpers need it.
+#DISCOURSE_SSO_SECRET=replace-me