Feat: add appendFile tool

[abusnitsky]

What does this PR do?

Adds a new appendFile tool so the agent can append content to the end of an existing file instead of overwriting it.

This PR includes:

• New appendFile implementation in functions.js
• Tool registration so the agent can call it from tools.js
• System prompt example for appendFile usage in agent.js
• Available tools documentation update in README.md

Type of change

• Bug fix
• New feature / tool
• Documentation update
• Refactor

How to test it

1. Start the CLI agent:

   • Run: node src/index.js

2. Create a test file with initial content:

   • Prompt: write "hello world" to test.txt

3. Ask the agent to append content:

   • Prompt: add "hello moon" to test.txt

4. Verify content was appended, not overwritten:

   • Prompt: show content of test.txt

Related issues

Closes #42

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[HoussemEddineChaouch]

Hey @abusnitsky, found a bug before merging

When appendFile is called on a non-existent file the
agent falls back and leaks a raw TOOL JSON string as
the final response instead of executing it:

Agent > TOOL {"name":"writeFile","args":...}-{"Reason":"..."}

Two things to fix:

1. appendFile should create the file if it doesn't
   exist.

2. The raw TOOL leak is a separate known bug but
   please make sure appendFile doesn't trigger it
   by handling the missing file case cleanly

Fix those before I merge. Thanks!

[abusnitsky]

@HoussemEddineChaouch Done. appendFile can now create a new file and doesn't trigger TOOL leak.