feat(mcp): gittensory_validate_linked_issue

[JSONbored]

Parent roadmap: #525
Parent phase: #544

Background

Miners chase the linked-issue scoring multiplier blind — they often link an issue that won't actually qualify (closed, multi-owner, not solved-by-this-PR), wasting the bonus.

Goal

New MCP tool gittensory_validate_linked_issue that reports whether the linked-issue multiplier will actually apply.

Current Behavior

The multiplier decision is computed only inside buildScorePreview; there's no standalone validator.

Desired Behavior

Input { owner, repo, issueNumber, plannedChange }. Returns whether the issue is a valid, open, single-owner, solvable-by-this-PR target and whether the multiplier would apply, with the blocking reason if not. Reuses the linked-issue logic in buildScorePreview (scoring/preview.ts:181) + buildIssueDiscoveryLifecycleReport.

Implementation Requirements

• Register in src/mcp/server.ts + package bin; outputSchema; metadata only.

Public/Private Output Boundaries

• Public-safe; the raw multiplier value stays private (return applies/does-not-apply + reason).

Acceptance Criteria

• Correctly flags valid vs invalid linked issues with the precise blocking reason.

Testing Requirements

• npm run test:ci, 97%+ coverage. Fixtures for valid/closed/multi-owner/unsolved cases.

[RealDiligent]

Follow-up: linked-issue extractor parity fix is ready on \RealDiligent:fix/critical-issue-mcp-linked-issue-extractor-parity\ (rebased on current \main\ @ bc1f54c, 63 targeted tests pass). Ports server-side #4039 inline code-span rejection and #3862 qualified \owner/repo#N\ scoping into MCP \extractLinkedIssues\ and \predicted-gate-engine\ — fixes the PR template (e.g. \\Closes #123\)\ false positive that also tripped Gittensory on prior PR #4057.

Blocked on a contributor-eligible upstream \gittensor:bug\ issue (none currently open). Could a maintainer please open one so we can resubmit with \Closes #…? RealDiligent cannot \CreateIssue\ here. Fork tracking: RealDiligent#4.