Switch agent structured contracts from JSON to XML tags; add XML builder and tests

[JohnEsleyer]

Motivation

• Standardize the agent structured contract format from JSON envelopes to an XML-tag based contract to simplify parsing and avoid JSON emission in prompts.
• Ensure the dashboard and agent test flows produce the new XML contract format when simulating file delivery and other actions.
• Provide a small, deterministic utility to build safe XML contracts and add tests to prevent regressions.

Description

• Updated examples and guidance in system_prompt.txt and docs/DOCKER_CUBICLES.md to use XML-tagged contracts (<thought>, <message>, <terminal>, <action>) instead of JSON envelopes.
• Modified the dashboard test prompt in dashboard/src/public/index.html to request XML-tagged contract output from agents.
• Added shell/src/xml-contract.ts implementing buildXmlContract with entity escaping and imported it into shell/src/server.ts to produce XML contracts in assistant history and to mark contractFormat as xml.
• Added a new route handler for XML contract tests in shell/src/server.ts that parses and logs XML contracts using existing parsing utilities.
• Added unit tests shell/tests/xml-contract.test.ts for buildXmlContract behavior and shell/tests/system-prompt-contract.test.ts to assert the system prompt no longer contains JSON examples and includes the XML tags.

Testing

• Ran the unit test suite with vitest and the new tests xml-contract.test.ts and system-prompt-contract.test.ts passed.
• Verified buildXmlContract outputs are detected as xml by detectContractFormat and are safely escaped by the tests.
• Confirmed changes to system_prompt.txt and docs are covered by the contract-format test and did not introduce test regressions.

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Codex Task