fix: only require TodoWrite when it is actually available

[Haohao-end]

Issue for this PR

Closes #18061

Type of change

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

What does this PR do?

This fixes a prompt/tooling mismatch around TodoWrite.

Before this change, the provider prompt could still tell the model to use TodoWrite even when todowrite was not actually available.

This change makes TodoWrite-specific prompt instructions conditional on the resolved tool set, so they are only included when todowrite is available.

This should work because the prompt now matches the tools the model can actually use.

How did you verify your code works?

I ran:

• bun test test/session/system.test.ts from packages/opencode

I also checked:

• git diff --check

Screenshots / recordings

N/A

Checklist

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

[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 #18202 - "fix(task): preserve subagent todo permissions"

• fix(task): preserve subagent todo permissions #18202
• This PR appears to address a similar issue about preserving subagent todo permissions, which directly relates to the second part of PR fix: only require TodoWrite when it is actually available #19057 that "preserves explicit todowrite / todoread access for subagents instead of always overriding it"

The current PR (19057) appears to be addressing a newer/refined version of this issue, closing #18061, but PR #18202 is worth reviewing to ensure there's no overlap or conflict.

[Haohao-end]

Scoped to #18061 only.

This PR only changes provider prompt generation so TodoWrite instructions are included only when todowrite is actually in the resolved tool set.

I intentionally dropped the subagent permission changes from this branch because that area overlaps with #18202.