feat: add package installation instructions to AGENTS.md during brin init

[homanp]

Summary

Closes #36

During brin init, after the docs index is written to AGENTS.md, also append a ## Package Installation section instructing agents to use brin instead of npm install, yarn add, pnpm add, or pip install.

The section is only added if it doesn't already exist (idempotent), so re-running brin init is safe.

Changes

• crates/cli/src/agents_md.rs: Added add_install_instructions() public function and internal add_install_instructions_at_path() with 3 new tests
• crates/cli/src/commands/init.rs: Call agents_md::add_install_instructions() after docs index step and print success message

Output after change

$ brin init

  🔧 initializing brin...

  Enable AGENTS.md docs index for AI coding agents? (Y/n) Y

  ✓ created brin.json
  ✓ created .brin-docs/
  ✓ updated AGENTS.md with brin docs index
  ✓ added package installation instructions to AGENTS.md

  ✓ brin initialized successfully!

[homanp]

@cursor review

[cursor]

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