fix(opencode): bound compaction request payload

[timrichardson]

Issue for this PR

Closes #29857

Type of change

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

What does this PR do?

Prevents /compact from creating a summarization request that is larger than the selected model's usable context budget.

The compaction path now:

• strips reasoning parts from compaction input
• strips media from compaction input as before
• adapts retained tool output size to the model budget
• treats toolOutputMaxChars: 0 as a real truncation limit

This fixes large restored sessions where the compaction request itself could overflow before a summary was produced.

How did you verify your code works?

• bun test test/session/message-v2.test.ts
• bun test test/session/compaction.test.ts
• bun run typecheck
• push hook also ran repo typecheck successfully

I also verified the anonymized fixture from #29857 locally:

• legacy payload shape: 788327 estimated tokens, overflows
• fixed payload shape: 308291 estimated tokens, compacts successfully

Screenshots / recordings

N/A

Checklist

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