fix: avoid duplicating cache_control when splitting identity system entry (#131)

## Summary

- When the identity prefix is split from a concatenated system entry,
`cache_control` was copied to **both** the identity block and the
remainder block
  via the spread operator
- This causes the Anthropic API to reject with `"A maximum of 4 blocks
with cache_control may be provided"` when users already have context
files
(CLAUDE.md, project docs, etc.) using the 4 allowed cache_control slots
- Now only the remainder block preserves `cache_control`; the identity
block is sent without it

  ## Reproduction

  1. Use OpenCode with `opencode-claude-auth` plugin
2. Load multiple context files with `cache_control` (e.g. CLAUDE.md +
project docs that fill all 4 slots)
3. Send any request → API returns 400: `"A maximum of 4 blocks with
cache_control may be provided. Found 5."`

  ## Fix

  ```diff
    const { text: _text, ...entryProps } = entry
  - splitSystem.push({ ...entryProps, text: SYSTEM_IDENTITY })
  + const { cache_control: _cc, ...identityProps } = entryProps
  + splitSystem.push({ ...identityProps, text: SYSTEM_IDENTITY })
```

  ## Test plan

  - Updated existing test to verify identity block does NOT have cache_control
  - Remainder block still preserves cache_control
  - All 26 existing tests pass