feat(mcp): gittensory_lint_pr_text (commit/PR-body rubric linter)

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Parent roadmap: #525
Parent phase: #544

Background

Low-effort / AI-slop PRs often have generic commit messages and empty PR bodies. A deterministic linter can catch this pre-submit and feeds the slop core (#530).

Goal

New MCP tool gittensory_lint_pr_text that scores a commit message + PR body against the gittensor traceability/no-issue-rationale rubric.

Current Behavior

No PR-text linter; the slop core (#530) lists "low-quality commit messages" as a signal but there is no standalone tool.

Desired Behavior

Input { commitMessages, prBody, linkedIssue? }. Returns a quality verdict + specific fixes. Reuses hasClearNoIssueRationale (engine.ts:3895), STOPWORDS (:672), and the traceability component of buildPublicReadinessScore (:3463).

Implementation Requirements

• Register in src/mcp/server.ts + package bin; outputSchema; deterministic. Share helpers with feat(signals): deterministic slop-assessment core #530.

Public/Private Output Boundaries

• Public-safe verdict + fixes only.

Acceptance Criteria

• Generic/empty text → low verdict with specific fixes; descriptive, issue-linked text → high.

Testing Requirements

• npm run test:ci, 97%+ coverage. Fixtures across template/generic/descriptive text.