fix: reject unknown ignore targets

[mikemikimike]

Summary

Reject unknown --targets values before scanning or writing ignore files.

Problem

An invalid target such as bogus was silently filtered out, so --write could exit successfully without writing any ignore file.

Changes

• Validate every non-empty target against the supported cursor, gemini, and generic targets.
• Return exit code 2 with the invalid values and valid target list before scanning or writing.
• Add a regression test covering an unknown target mixed with a valid target and bump the package version to 0.1.3 with a changelog entry.

Tests

• npm test (9 passed)
• node bin/ctxtrim.js test/fixtures/sample-repo --format json
• node --check src/cli.js
• node --check test/ctxtrim.test.js

Compatibility / Known limitations

Valid target names and existing report behavior are unchanged. Unknown targets now fail fast with exit code 2 and no ignore files are written.

Closes #9