Support bare provider template names (--provider anthropic) without a path #58

Feature

`--provider` currently requires either a full/relative path to a JSON template (`--provider templates/anthropic.json`) or `--provider generic --url ...`. There's no way to reference a template bundled with the installed package by its short name, so running from outside the repo always needs a full path to `pycodeloop/providers/templates/*.json`.

Fix

• `pycodeloop/providers/init.py` — added `BUNDLED_TEMPLATES` (set of template stems discovered from `providers/templates/*.json` at import time) and made `get_provider()` resolve a bare name in that set to the packaged JSON path before falling through to the `PROVIDERS` registry / `module:Class` syntax.
• `pycodeloop/cli/flow.py` — `resolve_provider()` now treats a bundled template name the same as a `.json` path for kwargs building, so it doesn't force-override the template's own `model` with `Settings.MODEL` when the user didn't pass `--model`.

Usage after the fix

```bash
pycodeloop --provider anthropic
pycodeloop --provider ollama
pycodeloop run "..." --provider openai --model gpt-5
```

Works from any directory once the package is installed — no path needed.