Add get_commands RPC for headless clients (#995)

* Add get_commands RPC for headless clients

Headless clients like Emacs can now query which commands are available.
Previously they could only discover file-based prompt templates by
scanning the filesystem; extension commands and skills were invisible.

The response includes each command's name, description, and source
(extension, template, or skill). Commands appear in the same order
as the TUI's autocomplete: extension commands first, then templates,
then skills.

Built-in TUI commands (/settings, /fork, etc.) are excluded since
they require the interactive UI. Commands like /compact have dedicated
RPC equivalents instead.

* Add location and path to get_commands response

Clients can show where commands come from (user/project/path) and
display file paths in tooltips. The data is already available on
templates and skills - just exposing it.

packages/coding-agent/docs/rpc.md

@@ -612,6 +612,47 @@ Response:
 
 Returns `{"text": null}` if no assistant messages exist.
 
+### Commands
+
+#### get_commands
+
+Get available commands (extension commands, prompt templates, and skills). These can be invoked via the `prompt` command by prefixing with `/`.
+
+```json
+{"type": "get_commands"}
+```
+
+Response:
+```json
+{
+  "type": "response",
+  "command": "get_commands",
+  "success": true,
+  "data": {
+    "commands": [
+      {"name": "session-name", "description": "Set or clear session name", "source": "extension", "path": "/home/user/.pi/agent/extensions/session.ts"},
+      {"name": "fix-tests", "description": "Fix failing tests", "source": "template", "location": "project", "path": "/home/user/myproject/.pi/agent/prompts/fix-tests.md"},
+      {"name": "skill:brave-search", "description": "Web search via Brave API", "source": "skill", "location": "user", "path": "/home/user/.pi/agent/skills/brave-search/SKILL.md"}
+    ]
+  }
+}
+```
+
+Each command has:
+- `name`: Command name (invoke with `/name`)
+- `description`: Human-readable description (optional for extension commands)
+- `source`: What kind of command:
+  - `"extension"`: Registered via `pi.registerCommand()` in an extension
+  - `"template"`: Loaded from a prompt template `.md` file
+  - `"skill"`: Loaded from a skill directory (name is prefixed with `skill:`)
+- `location`: Where it was loaded from (optional, not present for extensions):
+  - `"user"`: User-level (`~/.pi/agent/`)
+  - `"project"`: Project-level (`./.pi/agent/`)
+  - `"path"`: Explicit path via CLI or settings
+- `path`: Absolute file path to the command source (optional)
+
+**Note**: Built-in TUI commands (`/settings`, `/hotkeys`, etc.) are not included. They are handled only in interactive mode and would not execute if sent via `prompt`.
+
 ## Events
 
 Events are streamed to stdout as JSON lines during agent operation. Events do NOT include an `id` field (only responses do).