docs: add env command documentation to SKILL.md

Add section on Environment Variables for Agents:
- Explain env files (default.env, pm-agent.env)
- Document kugetsu env commands (list, show, set, get, rm)
- Show example usage for GITEA_TOKEN
- Note sensitive value masking
- Explain delegation usage

This ensures agents know to use kugetsu env instead of manually
injecting variables on each command.

skills/kugetsu/SKILL.md

@@ -48,6 +48,44 @@ A default config file is created during `kugetsu init` with commented examples:
 |----------|---------|-------------|
 | `MAX_CONCURRENT_AGENTS` | 3 | Maximum number of concurrent dev agents |
 
+### Environment Variables for Agents
+
+Agents receive environment variables through env files, not command-line injection. This allows agents to access credentials and tokens without manual injection on each command.
+
+**Files created during `kugetsu init`:**
+- `~/.kugetsu/env/default.env` - Variables for all agents
+- `~/.kugetsu/env/pm-agent.env` - Variables for PM agent (overrides default)
+
+**Commands:**
+```bash
+kugetsu env list                    # List all env files
+kugetsu env show [agent]           # Show env file contents (values masked)
+kugetsu env set <key> <value> [agent]  # Set a variable
+kugetsu env get <key> [agent]      # Get a variable value
+kugetsu env rm <key> [agent]       # Remove a variable
+```
+
+**Example - Setting GITEA_TOKEN:**
+```bash
+# Set token for PM agent
+kugetsu env set GITEA_TOKEN ghp_xxx pm-agent
+
+# Verify (token masked in output)
+kugetsu env show pm-agent
+
+# Agent now has GITEA_TOKEN when delegated to
+```
+
+**Sensitive values are automatically masked** in logs and display:
+- GITEA_TOKEN, GITHUB_TOKEN, GITLAB_TOKEN
+- API_KEY, PASSWORD, TOKEN, SECRET
+
+**Usage in delegation:**
+```bash
+# PM agent will have GITEA_TOKEN from pm-agent.env
+kugetsu delegate "post comment on #69"
+```
+
 ## Architecture
 
 ### Session Pattern