refactor(dual-franka): separate task policies from robot prompts

[NewYork424]

Description

Keep demo-specific manipulation policies in task definitions rather than shared robot prompts.

• Remove mandatory three-segment VLA, placement staging and advisory joint-recovery rules from shared prompts; retain controller safety.
• Clarify that fixed VLA policy conditioning is not retargeted by the planner's prompt.
• Update Task5 to require the spoon inside the cup, and add Task6 for two-cup dirty/clean sorting without chopsticks or spoons.

Scene-specific camera, ordering and destination requirements remain in the demo tasks. Operator reset and final-verdict requirements are unchanged.

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[NewYork424]

Superseded by #265 with identical commits and code changes. The source branch has moved to jx-qiu/RPent to keep the upstream repository's branch list clean. Review history remains here; please continue review on #265.