fix(tui): Don't overwrite the agent that was specified on the command line

[CarloWood]

If both -s and --agent are given, we should pick the agent from the latter.

Issue for this PR

Closes #20536

Type of change

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

What does this PR do?

At this point the command line arguments are already known; so we can simply check
for the existence of args.agent (with const args = useArgs()) to know if an --agent
command line parameter exists. If it does then the agent found in the last user message
of the given session ID -- that previously was unconditionally used to overwrite whatever
was in local.agent -- is no longer used. We simply keep the agent value that was
specified on the command line.

How did you verify your code works?

I repeated the 'Steps to reproduce' steps of #20536.
I can now specify -s and --agent at the same time and pick any agent I want.

Checklist

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

[github-actions]

Hey! Your PR title Don't overwrite the agent that was specified on the command line doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found one potentially related PR:

PR #18586: "fix(tui): prevent subagent sessions from overwriting the selected agent"

• fix(tui): prevent subagent sessions from overwriting the selected agent #18586

This PR appears to address a similar issue where agent selection was being unintentionally overwritten in certain scenarios (specifically for subagent sessions in the TUI). Your current PR (#20554) handles the same type of problem but specifically for the command-line interface when both -s (session) and --agent parameters are provided.

These are likely addressing different code paths but the same underlying concern about preserving intentional agent selection.

[CarloWood]

PR #18586: "fix(tui): prevent subagent sessions from overwriting the selected agent"

* [fix(tui): prevent subagent sessions from overwriting the selected agent #18586](https://github.com/anomalyco/opencode/pull/18586)

This PR appears to address a similar issue where agent selection was being unintentionally overwritten in certain scenarios (specifically for subagent sessions in the TUI). Your current PR (#20554) handles the same type of problem but specifically for the command-line interface when both -s (session) and --agent parameters are provided.

These are likely addressing different code paths but the same underlying concern about preserving intentional agent selection.

I checked #18586 and it is unrelated (it deals with subagents, not the collision between -s and --agent), although also making changes close to where I made a changes, in the same file. What it is doing will not be influenced by this patch (or the other way around).