fix: filter phantom package paths from ResolvePackagePaths

[em-redhat]

Summary

Fixes #104 -- loader.ResolvePackagePaths did not check pkg.Errors on
packages returned by go/packages.Load. When a pattern doesn't resolve to a
real package, go/packages returns a synthetic *packages.Package with
PkgPath set to the input pattern and Errors populated. The filter only
rejected empty/duplicate/_test paths, so phantom paths were returned as valid.

This fix adds pkg.Errors filtering and an io.Writer parameter for warning
output, following the project's established stderr injection pattern.

How to Test

# Run the new regression test (requires go/packages.Load -- no -short flag):
go test -race -count=1 -run TestResolvePackagePaths_InvalidPattern ./internal/loader/...

# Run the integration-level regression test that was previously failing:
go test -race -count=1 -run TestBuildContractCoverageFunc_InvalidPattern ./internal/crap/...

# Full short test suite:
go test -race -count=1 -short ./...

How to Demo

This is an internal bug fix. The observable change is that gaze analyze,
gaze quality, gaze crap, and gaze report now emit warnings to stderr
when a package pattern fails to resolve, instead of silently passing phantom
paths downstream.

# Before: nonexistent pattern silently returned phantom path
# After: nonexistent pattern is filtered with warning
gaze analyze github.com/nonexistent/pkg 2>&1
# Expected: "warning: skipping github.com/nonexistent/pkg: ..."

Key Files Changed

• internal/loader/loader.go -- Core fix: ResolvePackagePaths gains io.Writer param and pkg.Errors filtering with nil-safe warning output
• internal/loader/loader_test.go -- New TestResolvePackagePaths_InvalidPattern + 3 existing test signature updates
• cmd/gaze/main.go -- 2 callers updated (runAnalyze, runQuality) to pass p.stderr
• internal/aireport/runner_steps.go -- 2 callers updated (runQualityStep passes stderr, runClassifyStep passes nil per D4)
• internal/crap/contract.go -- 1 caller updated (BuildContractCoverageFunc passes stderr)
• openspec/changes/fix-phantom-package-paths/ -- Proposal, design, spec, and task artifacts

This PR was generated by /finale (AI-assisted).