feat(mcp): add miner planning prompts

[JSONbored]

Parent roadmap: #127
Parent phase: #269
Parent epic: #273

Goal

Add MCP prompts for miner planning workflows that guide humans through safe contribution decisions.

Requirements

• Add prompts for issue selection, PR packet drafting, branch preflight, and cleanup-first planning where supported.
• Keep prompts advisory and human-approved.
• Avoid requesting secrets, wallets, hotkeys, tokens, or private scoreability context.

Acceptance criteria

• MCP clients can discover miner planning prompts.

• Prompt text preserves public/private safety boundaries.

• Prompts do not create issues, PRs, comments, labels, closures, or merges.

• This is not public scoring or reward prediction.

• This is not an autonomous PR bot and not public scoring.

Validation expected

• MCP prompt discovery fixtures.
• Safety tests for forbidden request/claim language.
• npm run build:mcp.
• npm run test:mcp-pack.
• npm run test:ci.