fix(claude-code): add User-Agent header to prevent Cloudflare 403

[r266-tech]

Summary

Fixes #1041.

The HindsightClient in the Claude Code integration does not set a User-Agent header. Python's urllib defaults to Python-urllib/<version>, which Cloudflare blocks with error code 1010 ("browser signature banned"). This causes all auto-retain and auto-recall hooks to silently fail when the Hindsight server is behind Cloudflare or any reverse proxy with bot protection.

Changes

• Add User-Agent: hindsight-claude-code/1.0 to _headers() in client.py
• Add 2 regression tests verifying the header is present (with and without auth token)

Known limitations

• The version string 1.0 is hardcoded rather than derived from package metadata. This integration is a standalone script without formal versioning, so a constant is appropriate for now.
• The User-Agent value is not configurable per-instance. If a specific deployment needs a different UA string, this could be extended later.

Test plan

• New tests: TestHindsightClientUserAgent (2 tests)
• Existing tests unaffected (no behavior change for non-Cloudflare setups)

🤖 Generated with Claude Code