Update JUNIE_FILE path in update-agent-context scripts.

[Basster]

Thank you for merging PR #1831, but even though JUNIE supports an AGENTS.md file in the project root, the official path for the Junie Guidelines is .junie/guidelines.md, see https://www.jetbrains.com/guide/ai/article/junie/.

Description

Corrects the path to the official Junie guidelines.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

[Basster]

Nevermind, I was wrong and my knowledge was outdated. It's actually .junie/AGENTS.md. Source: https://junie.jetbrains.com/docs/guidelines-and-memory.html#how-junie-cli-discovers-guidelines

Closing.