fix(opencode): prevent context overflow during compaction

[Hexecu]

Target --> fix for : #11450

Using opencode often leads to exceeding token limits, even /compact fails to clean it up, getting the whole conversation stuck if the history is already too long

This PR adds
truncateForCompaction()
to
compaction.ts
which:

• Calculates the available input budget (context limit - output reserve - prompt reserve).
• Keeps existing summary messages (critical context).
• Adds recent messages until the budget is full.
• Truncates older messages to ensure the compaction request itself succeeds.

Fixes #11450

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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

Based on the search results, there are several related PRs that address similar context overflow and compaction issues, but no exact duplicates of PR #11453:

Related PRs:

1. PR fix(session): prevent context overflow by adding safety margin to compaction check #6562 - fix(session): prevent context overflow by adding safety margin to compaction check

   • Earlier fix for context overflow prevention with safety margins

2. PR feat(opencode): add context overflow prevention with configurable thresholds #8810 - feat(opencode): add context overflow prevention with configurable thresholds

   • Adds configurable thresholds for overflow prevention

3. PR feat(opencode): trigger compaction earlier and add multi-file read #9656 - feat(opencode): trigger compaction earlier and add multi-file read

   • Related approach to handle compaction triggers earlier

4. PR feat(session): add custom compaction thresholds along with prevention of continuous compaction #10123 - feat(session): add custom compaction thresholds along with prevention of continuous compaction

   • Adds custom thresholds and prevents continuous compaction

5. PR fix(opencode): accumulate step tokens for compaction #10215 - fix(opencode): accumulate step tokens for compaction

   • Related token calculation for compaction decisions

6. PR fix: add prompt caching to session compaction - Issue #10342 #10380 - fix: add prompt caching to session compaction - Issue #10342

   • Prompt caching for compaction optimization

These PRs all address the broader problem of context overflow and compaction, though PR #11453 introduces a specific truncateForCompaction() function that appears to be a more targeted solution.

[Hexecu]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)

2. Add `Fixes #<number>` or `Closes #<number>` to this PR description

See CONTRIBUTING.md for details.

Fixes #11453 added