fix(signals): diff all registry config fields in change reports

[philluiz2323]

Closes #280.

Problem

buildRegistryChangeReport diffed only 5 of the 8 comparable RegistryRepoConfig fields, omitting fixedBaseScore, defaultLabelMultiplier, and eligibilityMode. A repo whose only change is one of those fields produced an empty changes array and was dropped from changedRepos, so the report said 0 changed even though scoring materially changed (fixedBaseScore overrides the entire base score, preview.ts:287). The authoritative drift comparator (REGISTRY_DRIFT_COMPARABLE_FIELDS in upstream/ruleset.ts:606-615) already compares all 8 fields; the live report had drifted from it.

This is public-facing: GET /v1/registry/changes (routes.ts) and the registry-changes MCP tool.

Fix

Replace the 5 hardcoded comparison branches with a single data-driven field-descriptor table (REGISTRY_CHANGE_FIELDS) covering all 8 comparable fields, rendered to stable strings and diffed in one loop (registryConfigChanges). This adds fixed_base_score, default_label_multiplier, and eligibility_mode to the report and keeps the live report and the upstream drift comparator from diverging again as config fields are added. Existing change-line output for the original fields is preserved.

Tests

Added a fail-on-revert test: a snapshot pair differing only in fixedBaseScore (2 -> 50) must yield a changedRepos entry with fixed_base_score 2 -> 50 and a 1 changed summary; and eligibility_mode / default_label_multiplier changes are likewise surfaced. The existing test only exercised the 5 originally-compared fields, ratifying the omission.

vitest run signals-v2 + signals + registry 61/61; integration api 21/21; tsc --noEmit clean.

[JSONbored]

@philluiz2323 the change looks useful and CI is green, but the PR title needs to match the repo convention.

Required change:

• Retitle this to something like fix(signals): diff all registry config fields in change reports.

Validation expected:

• Keep npm run test:ci green.

[philluiz2323]

@philluiz2323 the change looks useful and CI is green, but the PR title needs to match the repo convention.

Required change:

• Retitle this to something like fix(signals): diff all registry config fields in change reports.

Validation expected:

• Keep npm run test:ci green.

done

[JSONbored]

@philluiz2323 the change may be small, but the PR still needs to meet the repo gate.

A few notes:

• Registry config diff coverage is a reasonable fix.
• The current title has a trailing period. (I fixed this)
• The body is missing current template evidence. (it's fine for now).

Required changes:

• None for now.

Validation expected:

• Focused signal tests.
• Full CI green.