Fix TypeError crashes for unknown agent and command names

[Haohao-end]

Issue for this PR

Closes #18310

Type of change

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

What does this PR do?

This fixes the unknown agent / unknown command TypeError path in session prompt flows.

Previously, Agent.get() / Command.get() could return undefined, and some must-exist prompt paths later dereferenced those values and crashed with generic TypeErrors.

This PR keeps the existing nullable get() behavior, adds typed not-found errors plus must() helpers, and switches only the required prompt paths to use them. It also keeps intentionally nullable paths unchanged.

How did you verify your code works?

I added regression tests for:

• unknown prompt agent
• unknown command

I also verified the affected prompt paths now fail with typed not-found errors instead of raw TypeErrors.

Screenshots / recordings

N/A

Checklist

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

[github-actions]

Hey! Your PR title Fix TypeError crashes for unknown agent and command names 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:

I found a potential related PR:

PR #18280 - "fix: improve plugin system robustness — agent/command resolution, async errors, hook timing, two-phase init"
#18280

Why it might be related: This PR also addresses agent/command resolution issues and robustness improvements in the plugin system, which overlaps with the scope of PR #18592's handling of unknown agent and command names. Both appear to be working on similar resolution and error handling paths.

However, note that PR #18592 is the current PR you're checking, and it specifically closes issue #18310. The other PR (#18280) may be addressing different aspects of the same system or related issues.