test: add CLI integration tests with assert_cmd

[rocketman-code]

Context

Parent: #160

The binary crate has ~320 lines of CLI dispatch logic in main.rs with ~1% test coverage. Functions like run_trace(), run_diff(), run_packages(), build_snapshot_from_working_tree(), and build_snapshot_from_ref() have zero tests. Three of eleven v0.4.0 bugs lived here (#142, #148, #150).

Expected Behavior

End-to-end tests that invoke the chainsaw binary with real arguments and assert on stdout, stderr, and exit codes.

Scope

• Add assert_cmd (and optionally predicates) as dev-dependencies
• Create tests/cli.rs integration test file
• Cover at minimum:
  • trace <entry> produces expected output format
  • trace --json <entry> produces valid JSON with expected field names
  • trace --chain <entry> <target> finds import chains
  • trace --cut <entry> <target> finds cut points
  • trace --max-weight <n> <entry> returns error when exceeded
  • diff <ref> with --entry produces diff output
  • diff --json <ref> --entry <entry> produces valid JSON
  • packages <entry> lists packages
  • packages --json <entry> produces valid JSON
  • Invalid flag combinations return non-zero exit code with helpful error message
  • Missing entry file returns non-zero exit code
• Use a small fixture project (2-3 .ts files) committed under tests/fixtures/