Document every exported symbol and how callers use the library.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/guide.md

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+# go-usertoken guide
+
+How to use the library. Symbol signatures and doc comments are in [API](api.md). Words for the domain are in [CONTEXT.md](../CONTEXT.md). Claim layout and the full environment list are in Knowledge `platforms/nextcloud/exapps/authentication.md`.
+
+## ExApp
+
+### Purpose
+
+The ExApp mints a short-lived RS256 token for the user AppAPI already named. Only the ExApp holds the private key. Microservices hold the matching public key, or they trust an OIDC issuer instead.
+
+### When to use it
+
+Mint a token when the ExApp calls a Microservice on behalf of the requesting user. Do not mint inside a Microservice.
+
+### Call sequence
+
+1. Generate an RSA key once and keep the private key on the ExApp. Copy `user-token.pub` to each Microservice that uses static mode.
+
+```bash
+openssl genrsa -out user-token.key 2048
+openssl rsa -in user-token.key -pubout -out user-token.pub
+```
+
+2. Set the ExApp environment:
+
+| Variable | Role |
+| --- | --- |
+| `USER_TOKEN_PRIVATE_KEY_FILE` | RSA private key PEM (PKCS#1 or PKCS#8) |
+| `USER_TOKEN_ISSUER` | `iss` string |
+| `USER_TOKEN_AUDIENCE` | `aud` string |
+| `USER_TOKEN_TTL` | optional, default `5m` |
+
+3. Call `NewSignerFromEnv` at startup, or `NewSigner` with a `SignConfig`.
+4. After AppAPI has named the user, call `Signer.Mint` with the current time and a `MintInput`.
+5. Send `Authorization: Bearer` plus the returned string.
+
+`MintInput.Subject` becomes both `sub` and `preferred_username`. `Groups` nil omits the groups claim. A non-nil pointer includes the claim, including an empty list.
+
+A `SignConfig.TTL` of zero uses 5 minutes, same as an empty `USER_TOKEN_TTL`.
+
+### Errors
+
+`NewSignerFromEnv` returns an error when the private-key path is empty, the PEM is not an RSA key, `USER_TOKEN_TTL` is set and is not a `time.Duration`, or issuer or audience is empty.
+
+`NewSigner` returns an error when the private key is nil or issuer or audience is empty.
+
+`Mint` returns an error when the subject is empty or the key cannot sign. Configuration failures are not `ErrUnauthorized`. That sentinel is for failed checks on the Microservice side.
+
+### Example
+
+```go
+signer, err := usertoken.NewSignerFromEnv()
+if err != nil {
+    return err
+}
+raw, err := signer.Mint(time.Now(), usertoken.MintInput{
+    Subject: userID,
+    Groups:  &groupIDs, // nil omits groups
+})
+if err != nil {
+    return err
+}
+req.Header.Set("Authorization", "Bearer "+raw)
+```
+
+## Microservice
+
+### Purpose
+
+A Microservice checks the bearer and stores the caller on the request context. One process uses either a static public key or an OIDC issuer.
+
+### When to use it
+
+Wrap HTTP handlers, or install the gRPC interceptor, at process start. Forward the same raw JWT when this service calls another.
+
+### Call sequence
+
+1. Set exactly one mode.
+
+Static public key (tokens this ExApp minted):
+
+| Variable | Role |
+| --- | --- |
+| `USER_TOKEN_PUBLIC_KEY_FILE` | RSA public key PEM |
+| `USER_TOKEN_ISSUER` | expected `iss` |
+| `USER_TOKEN_AUDIENCE` | expected `aud` |
+| `USER_TOKEN_SKEW` | optional, default `1m` |
+
+OIDC issuer:
+
+| Variable | Role |
+| --- | --- |
+| `OIDC_ISSUER` | issuer URL that serves discovery |
+| `OIDC_AUDIENCE` | expected `aud`; empty skips the audience check |
+| `OIDC_GROUPS_CLAIM` | group array claim, default `groups` |
+| `USER_TOKEN_SKEW` | optional, default `1m` |
+
+2. Call `FromEnv` with a context. The context is used for OIDC discovery. Or call `New` with a `Config` that sets either `PublicKey` or `OIDCIssuer`.
+3. Wrap handlers with `Auth.Middleware`. For gRPC, use `Auth.UnaryServerInterceptor`. Native clients send metadata `authorization`. The HTTP gateway's forwarded header is `grpcgateway-authorization`.
+4. Inside a handler, read `CallerFromContext` and `BearerFromContext`.
+5. On an outbound call, set `Authorization` to `Bearer ` plus the raw string from `BearerFromContext`.
+
+`/health` is not authenticated. Any other HTTP path without a valid bearer is 401. A gRPC call without a valid bearer is `Unauthenticated`.
+
+`Caller.Groups` is nil when the token omitted the claim, and non-nil (possibly empty) when the claim was present.
+
+### Errors
+
+`FromEnv` and `New` return an error when both modes are set, when neither mode is set, when the public key file is unreadable, when `USER_TOKEN_SKEW` cannot be parsed, or when OIDC discovery fails. Those are configuration errors. They do not wrap `ErrUnauthorized`.
+
+`Verify` returns an error that wraps `ErrUnauthorized` when the token is missing, expired, signed by the wrong key, or has the wrong issuer or audience. `Middleware` and `UnaryServerInterceptor` turn that into 401 or `Unauthenticated` and do not call the handler.
+
+An empty `OIDCAudience` skips the audience check. Static mode always requires an audience.
+
+### Example
+
+```go
+auth, err := usertoken.FromEnv(ctx)
+if err != nil {
+    return err
+}
+handler = auth.Middleware()(handler)
+
+// inside a handler
+caller, ok := usertoken.CallerFromContext(r.Context())
+if !ok {
+    http.Error(w, "unauthorized", http.StatusUnauthorized)
+    return
+}
+raw, _ := usertoken.BearerFromContext(r.Context())
+out.Header.Set("Authorization", "Bearer "+raw)
+_ = caller
+```