Add /context command and TestFixerAgent

[vivekmaru]

This PR introduces two features to improve developer experience and agent performance:

1. /context Command: Allows users to explicitly "pin" files to the agent's context using /context add <file>. Pinned files are injected into the system prompt on every turn, ensuring the agent doesn't lose track of critical files during long sessions.
2. TestFixerAgent: A specialized sub-agent (available as the test_fixer tool) designed to autonomously run a test command, analyze the output, and attempt to fix failures in a loop.

Changes:

• Modified Config to store pinnedFiles.
• Updated getCoreSystemPrompt to read and include pinned file contents.
• Added contextCommand implementation.
• Added TestFixerAgent definition and registration.

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PR created automatically by Jules for task 875954530216398720 started by @vivekmaru

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