Harden prompt context by excluding README and AGENTS

[ganesh47]

Motivation

• Prevent attacker-controlled root docs from being automatically embedded in LLM prompts by removing README.md and AGENTS.md from the default repository-context candidate list.

Description

• Remove "AGENTS.md" and "README.md" from REPOSITORY_CONTEXT_CANDIDATES in src/prompt.ts so only curated project documents are considered for prompt context.
• Add assertions to test/prompt.test.ts to verify the workflow prompt does not contain README excerpts and that the context reports Reference files: none when only README.md exists.

Testing

• Ran npm test -- test/prompt.test.ts and the test file passed successfully.

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Codex Task