fix: remove unused buildCanonicalFrontmatter function

[Copilot]

golangci-lint's unused checker flagged buildCanonicalFrontmatter in pkg/parser/frontmatter_hash.go, breaking the lint-go CI job. The function lost all its call sites when the parser was refactored to use the text-based hashing approach (computeFrontmatterHashTextBasedWithReader).

Changes

• pkg/parser/frontmatter_hash.go — deleted the dead buildCanonicalFrontmatter function (101 lines); the text-based hashing path fully replaces its role.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[CI Failure Doctor] CI Failure Investigation - Run #38509</issue_title>
<issue_description>### Summary
The lint-go job failed because golangci-lint reported the helper buildCanonicalFrontmatter in pkg/parser/frontmatter_hash.go as unused even though the rest of the parser no longer calls it. Removing this dead function keeps the lint suite happy.

Failure Details

• Run: 22580580495
• Commit: 75f03da
• Trigger: push

Root Cause Analysis

golangci-lint unused flagged buildCanonicalFrontmatter because the function is still defined even though every call site went away in the recent refactor. The lint job fails before we hit any other checks.

Failed Jobs and Errors

• lint-go: pkg/parser/frontmatter_hash.go:43:6: func buildCanonicalFrontmatter is unused (unused)

Investigation Findings

- The failure occurs inside `make lint` when `golangci-lint` runs `unused`. - Removing `buildCanonicalFrontmatter` and its documentation entry resolves the unused symbol. - Attempted local validation (`GOTOOLCHAIN=local make fmt` and `make lint`) but both commands fail in this environment because Go 1.25 cannot be downloaded and `golangci-lint` is not installed; these constraints will need to be addressed when verifying locally.

Recommended Actions

• Delete any remaining references to buildCanonicalFrontmatter and ensure the parser does not reintroduce it.
• Re-run GOTOOLCHAIN=local make fmt and GOTOOLCHAIN=local make lint (or the full make agent-finish) once the Go 1.25 toolchain and golangci-lint are available in the environment.

Prevention Strategies

Before running future dead-code-removal batches in the parser, clean up the helper definitions as soon as deadcode reports them so golangci-lint does not trip over stale symbols.

AI Team Self-Improvement

When removing parser helpers, proactively run the golangci-lint suite (or at least make lint) in a Go 1.25 environment to catch unused-symbol failures before pushing.

Historical Context

No previous investigations with this exact failure signature were found in the recent CI history.

🩺 Diagnosis provided by CI Failure Doctor

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• expires on Mar 3, 2026, 2:47 PM UTC

Comments on the Issue (you are @copilot in this section)

• Fixes [CI Failure Doctor] CI Failure Investigation - Run #38509 #19239

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[Copilot]

Pull request overview

Removes dead code in the Go parser to fix golangci-lint (unused) failures caused by the now-unreferenced buildCanonicalFrontmatter helper.

Changes:

• Deleted the unused buildCanonicalFrontmatter function from pkg/parser/frontmatter_hash.go after the refactor to the text-based hashing path.

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