feat(ai): add PI_CACHE_RETENTION env var for extended prompt caching

Adds support for extended cache retention via PI_CACHE_RETENTION=long:
- Anthropic: 5m -> 1h TTL
- OpenAI: in-memory -> 24h retention

Only applies to direct API calls (api.anthropic.com, api.openai.com).
Proxies and other providers are unaffected.

fixes #967

packages/coding-agent/README.md

@@ -525,6 +525,7 @@ pi --thinking high "Solve this complex problem"
 |----------|-------------|
 | `PI_CODING_AGENT_DIR` | Override config directory (default: `~/.pi/agent`) |
 | `PI_SKIP_VERSION_CHECK` | Skip version check at startup |
+| `PI_CACHE_RETENTION` | Set to `long` for extended prompt cache (Anthropic: 1h, OpenAI: 24h) |
 | `VISUAL`, `EDITOR` | External editor for Ctrl+G |
 
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