feat(coding-agent): add --no-extensions flag to disable extension discovery

Adds --no-extensions CLI flag that skips automatic extension discovery
while still allowing explicit -e paths. Three modes now available:

1. Default: auto-discover + any -e additions
2. --no-extensions: no extensions at all
3. --no-extensions -e foo.js: only load explicit extensions

Useful for debugging or running subagent instances without auto-discovered
extensions.

closes #524

packages/coding-agent/README.md

@@ -1125,6 +1125,7 @@ pi [options] [@files...] [messages...]
 | `--tools <tools>` | Comma-separated tool list (default: `read,bash,edit,write`) |
 | `--thinking <level>` | Thinking level: `off`, `minimal`, `low`, `medium`, `high` |
 | `--extension <path>`, `-e` | Load an extension file (can be used multiple times) |
+| `--no-extensions` | Disable extension discovery (explicit `-e` paths still work) |
 | `--no-skills` | Disable skills discovery and loading |
 | `--skills <patterns>` | Comma-separated glob patterns to filter skills (e.g., `git-*,docker`) |
 | `--export <file> [output]` | Export session to HTML |