fix(claude-code): support retain tags env override

[koriyoshi2041]

Summary

• add the missing HINDSIGHT_RETAIN_TAGS override to the Claude Code config loader
• cover comma-separated tag parsing and precedence through load_config()
• document the override in the integration README and synced docs copies

Tests

• uv run pytest hindsight-integrations/claude-code/tests/test_config.py (22 passed)
• uv run ruff check hindsight-integrations/claude-code/scripts/lib/config.py hindsight-integrations/claude-code/tests/test_config.py
• uv run ruff format --check hindsight-integrations/claude-code/scripts/lib/config.py hindsight-integrations/claude-code/tests/test_config.py
• git diff --check

Closes #2896