chore: remove dead functions (phase 8) — parser import system

[dsyme]

Removes 4 dead functions from the parser import system:

• ImportError.Error() and ImportError.Unwrap() — struct is still used but never cast to error interface; methods were dead from production paths
• ProcessImportsFromFrontmatter — thin wrapper now only needed in one test file, moved to local test helper
• ProcessImportsFromFrontmatterWithManifest — thin wrapper; all 16 test call sites updated to call ProcessImportsFromFrontmatterWithSource directly

Part of systematic dead code cleanup (DEADCODE.md phase 8).

[Copilot]

Pull request overview

Removes dead/unused functions from the parser import system and updates call sites (mostly tests) to use the remaining ProcessImportsFromFrontmatterWithSource API.

Changes:

• Removed dead ImportError.Error() / ImportError.Unwrap() methods and deleted wrapper import-processing functions from production code.
• Updated tests to call ProcessImportsFromFrontmatterWithSource directly (and introduced a local test helper where needed).
• Deleted now-obsolete unit test that only exercised the removed ImportError.Error() behavior.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
pkg/parser/import_processor.go	Removes the exported wrapper functions, leaving ProcessImportsFromFrontmatterWithSource as the entry point.
pkg/parser/import_error.go	Removes dead ImportError methods (Error, Unwrap).
pkg/parser/import_error_test.go	Removes unit test that only covered the deleted ImportError.Error() method.
pkg/parser/frontmatter_utils_test.go	Adds a local test helper to preserve the old wrapper behavior for this test file.
pkg/parser/yaml_import_test.go	Updates YAML-import tests to call ProcessImportsFromFrontmatterWithSource.
pkg/parser/yaml_import_e2e_test.go	Updates E2E YAML-import tests to call ProcessImportsFromFrontmatterWithSource.
pkg/parser/yaml_import_copilot_setup_test.go	Updates Copilot setup import test to call ProcessImportsFromFrontmatterWithSource.
pkg/parser/import_topological_test.go	Updates topological ordering tests to call ProcessImportsFromFrontmatterWithSource.
pkg/parser/import_topological_contract_test.go	Updates contract tests to call ProcessImportsFromFrontmatterWithSource.
pkg/parser/import_remote_nested_test.go	Updates remote/nested import test to call ProcessImportsFromFrontmatterWithSource.
pkg/parser/agent_import_integration_test.go	Updates agent import integration tests to call ProcessImportsFromFrontmatterWithSource.
pkg/cli/dependency_graph_test.go	Updates dependency-graph contract test to call ProcessImportsFromFrontmatterWithSource.

Comments suppressed due to low confidence (1)

pkg/parser/frontmatter_utils_test.go:552

• The test now calls the local helper processImportsFromFrontmatter, but several failure messages still refer to ProcessImportsFromFrontmatter() (which has been removed from production). Consider updating the strings to reference the helper / current API to avoid confusing test output.

			tools, engines, err := processImportsFromFrontmatter(tt.frontmatter, tempDir)

			if tt.wantErr {
				if err == nil {
					t.Errorf("ProcessImportsFromFrontmatter() expected error but got none")
				}

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[Copilot]

The helper comment and assertion message still refer to ProcessImportsFromFrontmatterWithManifest, but the helper now calls ProcessImportsFromFrontmatterWithSource. Please update this comment/message so the contract test documentation and failure output remain correct.

See below for a potential fix:

// ProcessImportsFromFrontmatterWithSource and returns the ImportedFiles slice.
func runImportProcessor(t *testing.T, dir string, topImports []string) []string {
	t.Helper()
	fm := map[string]any{"imports": topImports}
	result, err := parser.ProcessImportsFromFrontmatterWithSource(fm, dir, nil, "", "")
	require.NoError(t, err, "ProcessImportsFromFrontmatterWithSource must not fail")