docs: add per package README for implementation details (AI generated with human review)

agent/pkg/certs/README.md

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+# agent/pkg/certs
+
+Certificate management package for creating and extracting certificate archives.
+
+## Overview
+
+This package provides utilities for packaging SSL certificates into ZIP archives and extracting them. It is used by the GoDoxy Agent to distribute certificates to clients in a convenient format.
+
+## Architecture
+
+```mermaid
+graph LR
+    A[Raw Certs] --> B[ZipCert]
+    B --> C[ZIP Archive]
+    C --> D[ca.pem]
+    C --> E[cert.pem]
+    C --> F[key.pem]
+
+    G[ZIP Archive] --> H[ExtractCert]
+    H --> I[ca, crt, key]
+```
+
+## Public Functions
+
+### ZipCert
+
+```go
+func ZipCert(ca, crt, key []byte) ([]byte, error)
+```
+
+Creates a ZIP archive containing three PEM files:
+
+- `ca.pem` - CA certificate
+- `cert.pem` - Server/client certificate
+- `key.pem` - Private key
+
+**Parameters:**
+
+- `ca` - CA certificate in PEM format
+- `crt` - Certificate in PEM format
+- `key` - Private key in PEM format
+
+**Returns:**
+
+- ZIP archive bytes
+- Error if packing fails
+
+### ExtractCert
+
+```go
+func ExtractCert(data []byte) (ca, crt, key []byte, err error)
+```
+
+Extracts certificates from a ZIP archive created by `ZipCert`.
+
+**Parameters:**
+
+- `data` - ZIP archive bytes
+
+**Returns:**
+
+- `ca` - CA certificate bytes
+- `crt` - Certificate bytes
+- `key` - Private key bytes
+- Error if extraction fails
+
+### AgentCertsFilepath
+
+```go
+func AgentCertsFilepath(host string) (filepathOut string, ok bool)
+```
+
+Generates the file path for storing agent certificates.
+
+**Parameters:**
+
+- `host` - Agent hostname
+
+**Returns:**
+
+- Full file path within `certs/` directory
+- `false` if host is invalid (contains path separators or special characters)
+
+### isValidAgentHost
+
+```go
+func isValidAgentHost(host string) bool
+```
+
+Validates that a host string is safe for use in file paths.
+
+## Constants
+
+```go
+const AgentCertsBasePath = "certs"
+```
+
+Base directory for storing certificate archives.
+
+```go
+package main
+
+import (
+    "os"
+    "github.com/yusing/godoxy/agent/pkg/certs"
+)
+
+func main() {
+    // Read certificate files
+    caData, _ := os.ReadFile("ca.pem")
+    certData, _ := os.ReadFile("cert.pem")
+    keyData, _ := os.ReadFile("key.pem")
+
+    // Create ZIP archive
+    zipData, err := certs.ZipCert(caData, certData, keyData)
+    if err != nil {
+        panic(err)
+    }
+
+    // Save to file
+    os.WriteFile("agent-certs.zip", zipData, 0644)
+
+    // Extract from archive
+    ca, crt, key, err := certs.ExtractCert(zipData)
+    // ...
+}
+```
+
+## File Format
+
+The ZIP archive uses `zip.Store` compression (no compression) for fast creation and extraction. Each file is stored with its standard name (`ca.pem`, `cert.pem`, `key.pem`).