refactor(content-lane): retire the dead legacy candidate model

[JSONbored]

Summary

The metagraphed candidate-file model is fully superseded by the live surface model (#1340) and has zero src/ callers — only tests referenced it. This removes it:

• the legacy scope classifier: classifyPrScope, isDirectSubmissionScope, PrScope, ScopeResult (superseded by classifyRegistryPrScope / RegistryPrScope / RegistryScopeResult);
• the legacy validator assessCandidateDocument;
• the now-dead patterns CANDIDATE_PATTERN, PROVIDER_PATTERN, PROVIDER_ANY_PATTERN;
• the dead candidate dedup helpers candidateRegistryKey, registryDedupKeys, registryUrls;
• their index.ts re-exports and 34 now-orphaned tests.

KEPT (shared with the live surface validators): CandidateLike, Assessment, fail, ARTIFACT_PATTERN (used by METAGRAPHED_LANE_SPEC), and the secret/kind/url/observed-state checks (containsSecretLikeText, REVIEWER_SAFE_KINDS, isBaseLayerKind, OBSERVED_STATE_KEYS, isSafeEndpointUrl).

Validation

• Verified zero remaining references to the removed symbols repo-wide (src/ + test/ + packages/).
• Verified no orphaned internal helpers — the surface validators still call every shared helper.
• npm run typecheck clean; npm run test:ci green (109 registry-logic tests pass; 34 dead-symbol tests removed).
• ~430 lines of dead code removed; no behaviour change (pure deletion + comment rewording).

Follows #1340.

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.25%. Comparing base (d6748c4) to head (bb78788).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1342      +/-   ##
==========================================
- Coverage   95.26%   95.25%   -0.02%     
==========================================
  Files         188      188              
  Lines       20377    20314      -63     
  Branches     7347     7319      -28     
==========================================
- Hits        19413    19350      -63     
  Misses        378      378              
  Partials      586      586              

Files with missing lines	Coverage Δ
src/review/content-lane/registry-logic.ts	100.00% <ø> (ø)

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