fix(claude-code): preserve quoted channel envelopes

[koriyoshi2041]

Summary

• only unwrap Claude Code channel envelopes when they cover the complete message
• preserve user text when a channel envelope is quoted inside a larger prompt
• keep support for transport wrappers with surrounding whitespace

Tests

• uv run --no-sync pytest tests/test_content.py -q (59 passed)
• uv run --no-sync ruff check scripts/lib/content.py
• git diff --check

Closes #3124

[koriyoshi2041]

The generated-files check found the docs skill OpenAPI copy out of sync with the committed API schema. I regenerated and pushed c5b70ba; the only committed change is skills/hindsight-docs/references/openapi.json. The generation script is stable on a second run and git diff --check passes.