fix: prevent shell commands from killing all Node.js processes

[Xelson431]

Issue for this PR

Closes#7950

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

OpenCode is built on Node.js (node.exe). When the AI runs commands like taskkill /F /IM node.exe, killall node, pkill node, or Get-Process node | Stop-Process, it kills its own process and immediately crashes the assistant.

This PR adds defense in depth with three layers:

1. System prompt guard — Injects a critical safety reminder into the environment prompt for all models, instructing the AI never to run broad Node process-killing commands.
2. Shell tool prompt guard — Adds the same safety rule to the bash/shell tool description so the AI sees it every time the tool is invoked.
3. Runtime command guard — Validates shell commands before execution in ShellTool and throws a clear error if a dangerous pattern is detected.

Safer alternatives like targeting a specific PID (kill 12345) or using project-scoped stop commands (npm stop, pm2 stop <name>) remain unaffected.

How did you verify your code works?

Tested locally on Windows:

• Commands like taskkill /F /IM node.exe are blocked before execution
• Similar broad patterns (killall node, pkill node, Get-Process node | Stop-Process) are also rejected
• PID-based commands still work as expected
• Project-scoped stop commands continue to work
• Normal shell usage is unaffected for safe commands

Also reproduced the original issue before applying the fix, where killing all Node processes terminated the active OpenCode session.

Screenshots / recordings

N/A — this is not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[github-actions]

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