add Plano agent skills framework and rule set (#797)

* feat: add initial documentation for Plano Agent Skills

* feat: readme with examples

* feat: add detailed skills documentation and examples for Plano

---------

Co-authored-by: Adil Hafeez <adil.hafeez@gmail.com>

skills/rules/config-secrets.md

@@ -0,0 +1,72 @@
+---
+title: Use Environment Variable Substitution for All Secrets
+impact: CRITICAL
+impactDescription: Hardcoded API keys in config.yaml will be committed to version control and exposed in Docker container inspect output
+tags: config, security, secrets, api-keys, environment-variables
+---
+
+## Use Environment Variable Substitution for All Secrets
+
+Plano supports `$VAR_NAME` substitution in config values. This applies to `access_key` fields, `connection_string` for state storage, and `http_headers` in prompt targets and endpoints. Never hardcode credentials — Plano reads them from environment variables or a `.env` file at startup via `planoai up`.
+
+**Incorrect (hardcoded secrets):**
+
+```yaml
+version: v0.3.0
+
+model_providers:
+  - model: openai/gpt-4o
+    access_key: abcdefghijklmnopqrstuvwxyz...   # Hardcoded — never do this
+
+state_storage:
+  type: postgres
+  connection_string: "postgresql://admin:mysecretpassword@prod-db:5432/plano"
+
+prompt_targets:
+  - name: get_data
+    endpoint:
+      name: my_api
+      http_headers:
+        Authorization: "Bearer abcdefghijklmnopqrstuvwxyz"   # Hardcoded token
+```
+
+**Correct (environment variable substitution):**
+
+```yaml
+version: v0.3.0
+
+model_providers:
+  - model: openai/gpt-4o
+    access_key: $OPENAI_API_KEY
+    default: true
+
+  - model: anthropic/claude-sonnet-4-20250514
+    access_key: $ANTHROPIC_API_KEY
+
+state_storage:
+  type: postgres
+  connection_string: "postgresql://${DB_USER}:${DB_PASS}@${DB_HOST}:5432/${DB_NAME}"
+
+prompt_targets:
+  - name: get_data
+    endpoint:
+      name: my_api
+      http_headers:
+        Authorization: "Bearer $MY_API_TOKEN"
+```
+
+**`.env` file pattern (loaded automatically by `planoai up`):**
+
+```bash
+# .env — add to .gitignore
+OPENAI_API_KEY=sk-proj-...
+ANTHROPIC_API_KEY=sk-ant-...
+DB_USER=plano
+DB_PASS=secure-password
+DB_HOST=localhost
+MY_API_TOKEN=tok_live_...
+```
+
+Plano also accepts keys set directly in the shell environment. Variables referenced in config but not found at startup cause `planoai up` to fail with a clear error listing the missing keys.
+
+Reference: https://github.com/katanemo/archgw