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).

README.md

@@ -9,10 +9,12 @@ land). AGPLv3.
 Status: 2026-08-29 — v0 complete and green: categories (list/create),
 topics (create/list/latest/get), posts (create/update/get), current-user
 identity probe, raw JSON passthrough for everything else, typed error
-classes, thin CLI. Built and tested entirely against a fake Discourse
-(no live writes). Live reads verified against community.turnsys.com;
-category creation 403s with the current key (needs the admin-scoped key)
-— see "Live verification" below.
+classes, thin CLI. Same-day additions (Redmine 507): full-text search,
+private-message send, and the webhook + SSO (Discourse Connect) secret
+helpers. Built and tested entirely against a fake Discourse (no live
+writes). Live reads verified against community.turnsys.com; category
+creation 403s with the current key (needs the admin-scoped key) — see
+"Live verification" below.
 
 ## What it implements
 
@@ -30,6 +32,10 @@ The wire protocol, in plain REST with stdlib:
 | posts create | `POST /posts.json` — `topic_id`+`raw` |
 | posts update | `PUT /posts/<id>.json` — `{post: {raw, edit_reason}}` |
 | posts get | `GET /posts/<id>.json` |
+| search | `GET /search.json?term=...&page=N` — grouped posts/topics/users/categories hits; term operators (`@user`, `#category`, `order:latest`) pass through |
+| message send | `POST /posts.json` — `title`+`raw`+`target_usernames`/`target_group_names`/`target_emails` (comma-joined) instead of a category; creates a private-message topic |
+| webhook verify | `X-Discourse-Event-Signature` — sha256-HMAC hex (with `sha256=` prefix) over the raw body, constant-time compare |
+| SSO verify/response | Discourse Connect — sha256-HMAC hex over the base64 `sso` string; payload = urlencoded fields (nonce, return_sso_url / email, external_id, ...) |
 | raw anything | `Do(ctx, METHOD, path, body, out)` — JSON in, JSON out; unmodeled endpoints never block on a client release |
 
 Auth is the header pair `Api-Key` + `Api-Username` on every request.
@@ -55,8 +61,37 @@ res.URL(baseURL) // https://forum/t/<slug>/<topic_id>
 
 p, _ := c.CreatePost(ctx, discourse.CreatePostRequest{TopicID: res.TopicID, Raw: "reply"})
 _, _  = c.UpdatePost(ctx, p.PostID, "edited", "typo")
+
+hits, _ := c.Search(ctx, discourse.SearchRequest{Term: "fleet briefing", Page: 1})
+// hits.Posts[0].TopicID / .Blurb; also .Topics, .Users, .Categories
+
+pm, _ := c.SendMessage(ctx, discourse.SendMessageRequest{
+    Title: "Briefing landed", Raw: markdown,
+    TargetUsernames: []string{"ops", "reachableceo"},
+})
+pm.URL(c.BaseURL()) // private-message topic link
 ```
 
+### Webhook + SSO helpers (server side)
+
+```go
+// webhook receiver: verify before trusting anything
+http.Handle("/hooks/discourse", discourse.WebhookHandler(secret, func(evt *discourse.WebhookEvent) {
+    // evt.Event == "post_created" etc.; evt.Body is the raw JSON
+}))
+err := discourse.VerifyWebhook(secret, body, sigHeader) // standalone
+
+// Discourse Connect (SSO) login endpoint
+redirect, err := discourse.SSOLogin(ctx, secret, r.FormValue("sso"), r.FormValue("sig"),
+    map[string]string{"email": u.Email, "external_id": u.ID, "username": u.Handle})
+http.Redirect(w, r, redirect, http.StatusFound)
+
+// secrets: env-only, trimmed, never logged
+secret, _ := discourse.WebhookSecretFromEnv() // DISCOURSE_WEBHOOK_SECRET
+secret, _ := discourse.SSOSecretFromEnv()     // DISCOURSE_SSO_SECRET
+```
+Bad signatures are the typed `ErrBadSignature` — drop the delivery.
+
 Errors classify via `errors.Is`: `ErrForbidden` (403 — key lacks scope),
 `ErrUnauthorized` (401), `ErrNotFound`, `ErrRateLimited` (429, with
 `APIError.RetryAfter`), `ErrServer`, `ErrUnreachable`,
@@ -84,7 +119,8 @@ End-to-end smoke — builds the CLI, boots the fake Discourse in a
 container on `127.0.0.1:8610`, drives the real binary from the host
 through a 0600 env file (whoami / category list+create / topic create /
 topic list by id and slug / post reply / post update / raw passthrough /
-typed 404 / redaction sweep / bad-key exit code):
+search / private-message send / webhook verify good+bad sig / sso
+verify good+bad sig / typed 404 / redaction sweep / bad-key exit code):
 ```sh
 ./dev.sh smoke
 ```
@@ -99,7 +135,8 @@ Credentials NEVER arrive via flags or arguments:
 ```sh
 mkdir -p ~/.config/discourse-go && umask 077
 cp env.example ~/.config/discourse-go/env
-# fill in DISCOURSE_URL / DISCOURSE_API_KEY / DISCOURSE_API_USERNAME
+# fill in DISCOURSE_URL / DISCOURSE_API_KEY (or DISCOURSE_KEY) /
+# DISCOURSE_API_USERNAME, and the webhook/SSO secrets if used
 ```
 Then `set -a; . ~/.config/discourse-go/env; set +a` before the CLI (or
 source it in the service env). For the harness, the key resolves through
@@ -116,11 +153,16 @@ discourse-go topics show ID
 discourse-go topics create -title TITLE -category ID [-file PATH | -raw TEXT]
 discourse-go posts create -topic ID [-file PATH | -raw TEXT]
 discourse-go posts update ID [-file PATH | -raw TEXT] [-reason TEXT]
+discourse-go search TERM [-page N]
+discourse-go messages send -title TITLE -to USER[,USER...] [-group NAMES] [-email ADDRS] [-file PATH | -raw TEXT]
+discourse-go webhook verify SIG          # delivery body on stdin, secret from DISCOURSE_WEBHOOK_SECRET
+discourse-go sso verify SSO SIG          # secret from DISCOURSE_SSO_SECRET; prints the payload as JSON
 discourse-go raw METHOD PATH [-data JSON|@file]
 ```
 Perm levels: 1 = reply/see, 2 = create posts, 3 = full. Output is JSON.
 Exit codes: 0 ok, 1 usage/config, 2 API/transport error (the message
-names the class).
+names the class). `webhook verify` / `sso verify` need no instance
+credentials — only their secret.
 
 ## Live verification (community.turnsys.com, 2026-08-29)