fix: clean error when report --out parent dir is missing

[sharadvc]

What

ctxlens report --out missing/dir/r.html crashed with an unhandled FileNotFoundError traceback because out.write_text(...) sat outside _load's try/except.

Change

Wrap the report write in the same OSError handling _load already uses: print a clean error: line to stderr and exit with code 1. Mirrors the accepted error path used everywhere else in the CLI.

Tests

Added a failing test first (per the issue's acceptance criteria): report with --out pointing into a missing parent directory expects exit 1, a clean error: line, and no traceback. Full suite: 62 passed. ruff check clean.

Related

Closes #13