fix: impact analysis shows project-wide test_count instead of per-model affected tests

[anandgupta42]

Problem

impact_analysis reports manifest.test_count (total tests in the entire dbt project) as "Tests in project: 47", implying all 47 tests are at risk from the change. In reality, only a subset of tests reference the affected model.

Location

packages/opencode/src/altimate/tools/impact-analysis.ts:85-86

Current Behavior

Tests in project: 47
  Run `dbt test` to verify all 47 tests still pass after this change.

Expected Behavior

Either:

1. Filter to tests that actually reference the affected model/column (requires DbtManifestResult to expose individual test depends_on)
2. Or clarify the wording: "Total project tests: 47. Some may be affected — run dbt test to verify."

Found During

PR #350 deep analysis.