fix(explore): prevent .sisyphus shortcut for small-parameter models (fixes #2880)

[MoerAI]

Summary

• Add explicit constraint to explore agent prompt preventing .sisyphus plan file shortcuts.

Problem

Small-parameter models (Minimax-M2.5, M2.7) assigned explore tasks read .sisyphus/ plan and evidence files and immediately consider the task complete without actually searching the codebase. This causes missed findings during validation and review passes.

Fix

Added a "No shortcut via plan files" constraint to the explore agent prompt that explicitly prohibits reading .sisyphus/ directory files as a substitute for codebase search. The agent must use search tools (grep, glob, ast_grep, LSP) to find real source code.

Changes

File	Change
src/agents/explore.ts	Added .sisyphus/ exclusion constraint to prompt

Fixes #2880

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Summary by cubic

Stop the explore agent from shortcutting by reading .sisyphus/ plan and evidence files. Adds a prompt rule that requires using search tools (grep, glob, ast_grep, LSP) to find real code and fixes #2880.

^{Written for commit 63e0747. Summary will update on new commits.}

[cubic-dev-ai]

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[MoerAI]

I have read the CLA Document and I hereby sign the CLA

[code-yeongyu]

LGTM. Good guard -- small-parameter explore models (minimax, kimi etc.) are tempted to shortcut via .sisyphus/ plan files instead of actually searching the codebase. Making this explicit in the prompt is the right approach.