feat(preflight): generate public-safe PR packets from metadata only

[JSONbored]

Parent: #32

Goal

Make MCP output directly useful for drafting high-quality PR descriptions.

Requirements

• Generate a public-safe PR packet from branch metadata, linked issues, changed file summaries, validation summaries, duplicate risk, and lane context.
• Exclude private scoreability, reward/risk, raw trust, wallets, hotkeys, and reviewability internals.
• Support copy/paste output from CLI and MCP.
• Keep source upload unsupported.

Acceptance Criteria

• gittensory-mcp agent packet returns a maintainer-friendly packet.
• Packet includes tests run, linked issue state, duplicate/WIP caution, branch freshness, and next steps.
• Packet never contains private score/reward/trust language.
• Local metadata only; no source content.

Testing Requirements

This issue must preserve the repo's CI gate:

• npm run test:ci must pass.
• Global coverage must remain at or above 95% for lines, statements, functions, and branches.
• Add or update tests for every new branch, fallback path, sanitizer rule, and regression.
• Add invariant/property-style tests when behavior depends on sorting, gating, public/private boundaries, scoring, queue pressure, or source-upload safety.
• Public GitHub output must be tested against forbidden language: wallet, hotkey, raw trust score, payout, reward estimate, farming, private reviewability, and public score estimate.

Required Test Scenarios

• Clean branch packet.
• Missing tests packet.
• Linked issue packet.
• Duplicate risk packet.
• Stale base warning packet.
• Forbidden-language regression.
• Source-upload guard.