feat: add AssumeRoleActivity for temporary LLM API token grants

Implements AWS AssumeRole-like pattern for Poimen:
- User/service requests temporary access with identity + scope
- AssumeRoleActivity exchanges credentials with OAuth2 auth server
- Returns JWT token valid for limited time (default: 1hr, max: 24hrs)
- Token used in all subsequent LLM API calls to api.riotpiao.com

Key features:
- Credentials from vault/K8s secrets (never hardcoded)
- Scope-based access control (llm:read, llm:read llm:write, llm:admin)
- Automatic token expiration tracking
- Retry support for transient auth failures (2x, 1.5s backoff)
- Configurable auth server endpoint

Usage pattern:
1. AssumeRoleActivity(identity, scope) → JWT token
2. LLMRouter uses token in LLMAuth config
3. All activity calls validated against token + scopes
4. Workflow optionally refreshes token before expiry

Security:
- No credentials in code/logs (env or vault only)
- Short-lived tokens (1hr default, 24hr max)
- Server-enforced scope validation
- Token revocation support

Activity registered: #10 (authentication category)
Knowledge base updated with full activity spec

New file: action/assume_role.go (5.2 KB)

cmd/worker/main.go

@@ -85,6 +85,9 @@ func main() {
 	w.RegisterActivity(action.DeploymentPreCheckActivity)
 	w.RegisterActivity(action.ApproveWorkflowActivity)
 
+	// Authentication activities
+	w.RegisterActivity(action.AssumeRoleActivity)
+
 	// Memory activities
 	w.RegisterActivity(action.RetrieveMemoryActivity)