feat(agent): miner write-tools (open_pr/file_issue/...) — local-executed

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Parent epic: #770 (Phase 2).

Goal

New MCP write-tools that the user's local harness executes with their own GitHub creds (gittensory supplies content, never performs the write — preserves the boundary).

Deliverables

• open_pr (from gittensory PR-packet), file_issue, apply_labels, post_eligibility_comment
• create_branch, delete_branch helpers
• Clear local-execution contract (writes never go through gittensory cloud)

Context

packages/gittensory-mcp/bin (stdio), src/mcp/server.ts, PR-packet (src/signals/local-branch.ts).

Milestone: M4.