Add proxymock secrets guide

docs/proxymock/guides/secrets.md

@@ -0,0 +1,121 @@
+---
+description: "Use the ${{secret:name/key}} syntax to inject sensitive values like database credentials, JWT signing keys, and API tokens into proxymock transforms without storing them in your recording."
+sidebar_position: 8
+---
+
+# Secrets
+
+Recorded traffic often contains credentials that differ between environments — database passwords, JWT signing keys, AWS access keys. The `${{secret:name/key}}` syntax lets proxymock resolve these values from local files at runtime, so transforms can re-sign tokens or re-authenticate connections without hardcoding secrets into your recording or blueprint.
+
+## Directory layout
+
+proxymock discovers secrets automatically by walking up from your `--in` directory. Place a `secrets/` directory alongside (or inside) your `proxymock/` workspace:
+
+```
+my-project/
+├── proxymock/
+│   ├── secrets/                    ← preferred location
+│   │   ├── mongo-creds/
+│   │   │   ├── host
+│   │   │   ├── username
+│   │   │   └── password
+│   │   └── jwt-signing/
+│   │       └── key
+│   └── recorded-2026-06-01T120000Z/
+│       └── ...
+```
+
+Each secret is a plain file whose contents are the value. For example, `secrets/mongo-creds/password` might contain `s3cret123` with no trailing newline.
+
+An alternative layout places `secrets/` one level up from `proxymock/`:
+
+```
+my-project/
+├── secrets/
+│   └── mongo-creds/
+│       └── ...
+└── proxymock/
+    └── recorded-.../
+```
+
+Both layouts work. proxymock checks `<workspace>/proxymock/secrets/` first, then `<workspace>/secrets/`.
+
+## Referencing secrets in transforms
+
+Use the `${{secret:name/key}}` syntax in transform configuration fields that accept secret references. The `name` maps to the directory and `key` maps to the filename inside it.
+
+### MongoDB SASL authentication
+
+The `sasl_auth` transform re-authenticates MongoDB connections using SCRAM. Point each field at the appropriate secret file:
+
+```json
+{
+  "type": "sasl_auth",
+  "config": {
+    "host":     "${{secret:mongo-creds/host}}",
+    "username": "${{secret:mongo-creds/username}}",
+    "password": "${{secret:mongo-creds/password}}"
+  }
+}
+```
+
+This resolves to `secrets/mongo-creds/host`, `secrets/mongo-creds/username`, and `secrets/mongo-creds/password` on disk.
+
+For multiple MongoDB hosts, add one `sasl_auth` transform per host, each referencing its own secret directory.
+
+### JWT re-signing
+
+The `jwt_resign` transform re-signs JWT tokens using a local key:
+
+```json
+{
+  "type": "jwt_resign",
+  "config": {
+    "secretPath": "${{secret:jwt-signing/key}}"
+  }
+}
+```
+
+### AWS Signature V4
+
+The `aws_auth` transform re-signs AWS requests:
+
+```json
+{
+  "type": "aws_auth",
+  "config": {
+    "secretPath": "${{secret:aws-creds/secret}}",
+    "idPath":     "${{secret:aws-creds/id}}"
+  }
+}
+```
+
+## CLI usage
+
+No extra flags are needed. When you run `proxymock mock` or `proxymock replay`, the secrets directory is discovered automatically from the `--in` path:
+
+```bash
+proxymock replay --in ./proxymock/recorded-2026-06-01T120000Z
+```
+
+proxymock walks up from the `--in` directory looking for a `secrets/` folder. If it finds one, all `${{secret:name/key}}` references in your transforms resolve against it.
+
+## Security notes
+
+- **Never commit secrets to version control.** Add `secrets/` to your `.gitignore`.
+- Path traversal is blocked — references like `${{secret:../etc/passwd}}` are rejected and treated as a missing file.
+- In Kubernetes, the Speedscale operator mounts secrets from the cluster's secret store. The local `secrets/` directory is only used for out-of-cluster (CLI) runs.
+
+## Troubleshooting
+
+**Transform fails with "file not found" for a secret path**
+The `secrets/` directory wasn't discovered. Verify the directory exists and is reachable by walking up from your `--in` path. The directory must be named exactly `secrets` (not `.secrets` or `_secrets`).
+
+**Secret value has unexpected whitespace**
+Secret files are read as-is. Make sure the file doesn't contain a trailing newline. On macOS/Linux:
+
+```bash
+printf 's3cret123' > secrets/mongo-creds/password
+```
+
+Using `echo` adds a trailing newline by default — use `printf` or `echo -n` instead.