fix: generate error code catalog from single YAML source

[mysteriousskater]

Overview

This PR consolidates the duplicated error code documentation into a single hand-maintained source. docs/error-codes.yaml becomes the sole source of truth; the generator now emits both src/errors/codes.ts and docs/error-code-catalog.md, the hand-written docs/error-codes.md duplicate is removed, and npm run error-codes:check verifies the generated markdown is up to date.

Related Issue

Changes

🧾 Single Source of Truth

• [MODIFY] docs/error-codes.yaml

  • Remains the only hand-maintained error code source; no duplicate prose is kept elsewhere.

• [DELETE] docs/error-codes.md

  • Removed the hand-written duplicate that drifted from the YAML and the generated catalog.

• [MODIFY] scripts/generate-error-codes.mjs

  • Emits docs/error-code-catalog.md in addition to src/errors/codes.ts from the YAML source.
  • Adds a check mode so npm run error-codes:check fails when the generated markdown (or TypeScript) is stale.

• [MODIFY] docs/error-code-catalog.md

  • Now a generated artifact reflecting the YAML source.

• [MODIFY] package.json

  • Wires error-codes:check to the generator's check mode.

• [MODIFY] scripts/generate-error-codes.test.mjs

  • Adds coverage for markdown output generation and staleness detection.

Verification Results

npm run error-codes:check
node --test scripts/generate-error-codes.test.mjs

Acceptance Criteria	Status
Only one hand-maintained error code source remains	✅ docs/error-codes.yaml is the sole source; docs/error-codes.md removed
The markdown catalogue is generated	✅ Generator emits docs/error-code-catalog.md
error-codes:check fails when the markdown is stale	✅ Check mode compares generated output against committed files
scripts/generate-error-codes.test.mjs covers markdown output	✅ Tests assert markdown generation and staleness behavior

Security and Failure Modes

• The check mode fails closed: any drift between the YAML source and committed generated files causes a non-zero exit, so stale docs cannot silently pass CI.
• No validation or safeguards were weakened; the generator only adds an output target and a verification path.

Compatibility

• src/errors/codes.ts output is unchanged in shape, so existing imports and error handling continue to work.
• The removed docs/error-codes.md content is preserved through the generated docs/error-code-catalog.md.

Closes #1335

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