[Prompt/Skill] Add explicit publishability gate mode

[gaofeng21cn]

Summary

This PR makes publishability gating an explicit startup-contract policy for paper-required quests.

Changes included:

• add publishability_gate_mode parsing to the prompt builder
• support off | warn | enforce policy behavior in paper-mode delivery guidance
• align decision and write skill wording with the startup-contract policy
• document the new advanced startup-contract field in the English and Chinese start guides
• add prompt-builder tests for off and warn behavior, and update the long-horizon completion assertion

Why

DeepScientist already had strong publishability-oriented paper guidance, but it was effectively implicit. This PR makes the policy explicit and inspectable at startup-contract level without hard-coding domain- or venue-specific thresholds.

Scope

This PR intentionally does not:

• add venue-specific defaults such as "Q2+ only"
• add any domain-specific metric cutoffs
• add a new plugin system or controller framework
• expose the field in the start dialog yet

The new field is documented as an advanced startup-contract option.

Issue

Resolves #8

Tests

• pytest tests/test_prompt_builder.py

Docs

• updated docs/en/02_START_RESEARCH_GUIDE.md
• updated docs/zh/02_START_RESEARCH_GUIDE.md

AI Assistance

This PR was prepared with AI assistance and reviewed locally before submission.