fix(installation): detect install method from invoked CLI path

[vsiegel]

Issue for this PR

Closes #26479

Type of change

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

What does this PR do?

This fixes installation-method detection used by auto-upgrade. Previously the code used process.execPath (runtime executable like node/bun) as the primary signal, which can differ from the actual invoked opencode command path. In some local setups that can choose the wrong installer path during upgrade and leave the command unavailable after exiting.

The change switches method heuristics to inspect the invoked CLI path (process.argv[1] fallback chain) and keeps the existing curl path checks and package-manager prioritization behavior.

How did you verify your code works?

• Reproduced and analyzed the failing behavior path where runtime executable path mismatches command installation path.
• Validated the diff is scoped to install-method detection only.
• Attempted local typecheck in packages/opencode; environment is missing tsgo, so full typecheck could not run here.

Screenshots / recordings

N/A (non-UI change)

Checklist

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

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[vsiegel]

I installed opencode with "npm i -g opencode-ai@latest".
The command opencode repeatedly vanished, so often that it felt really weird not to find a critical issue about it.
Assuming it must be related to the local situation, I literally asked opencode to submit this PR with a fix.
If I get it right, opencode tries to update itself, misjudges how it was installed and proceeds to uninstall itself.

Admittedly, there is educational value in seeing an agent commit suicide.

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