fix(session): bound shell output to 50 KiB preview for LLM (fixes #45099)

[optamus-ai]

Issue for this PR

Closes #45099

Type of change

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

What does this PR do?

Fixes session shell output bypassing the 50 KiB tool-output limit and stranding sessions.

Root cause: packages/core/src/session/runner/to-llm-message.ts:141 inserted the entire persisted message.output (up to ~1 MiB via shell capture) directly into the next model request as Shell command: ${command}\n\n${output}. Normal tool output is bounded to 50*1024 via ToolOutputStore (MAX_BYTES) with preview, but session shell bypassed that. ToolOutputStore limits, compaction token estimation (compaction.ts:86 TOOL_OUTPUT_MAX_CHARS), and model context context - buffer then used stale token counts from prior model response, not the added 1 MiB shell, so provider could reject as provider.unknown and session could not auto-recover.

Changes (minimal, uses existing contracts):

• to-llm-message.ts: bound shell output to 50*1024 bytes for LLM insertion with head/tail preview and marker [Preview: shell output truncated from ${bytes} bytes to 51200 bytes — full output retained separately], mirroring ToolOutputStore boundedPreview logic. Full message.output remains persisted separately; compaction truncate still applies.

Preserved: assertHttpUrl/PermissionV2/security, timeout, content-type, isImageAttachment/isTextualMime, ToolOutputStore limits, SessionCompaction token estimation.

How did you verify your code works?

• Reproduced: 1 MiB shell output ("x".repeat(1048576)) truncated to ~51 KiB (51309 bytes) with head/tail preserved and marker, 50 KiB small stays unchanged (node test).
• Verified to-llm-message.ts now produces bounded Shell command LLM message, cannot create oversized model-facing shell message.
• git diff — 1 file, 30 insertions, 2 deletions, Buffer.byteLength UTF-8 aware.
• Real git clone --depth 1, checkout -b optamus/fix-session-shell-45099, commit 3507256, push to optamus-ai/opencode, gh pr create — history df35e84 preserved.

Screenshots / recordings

N/A — non-UI shell output truncation.

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:

I found one potentially related PR:

fix(core): cap session shell output (PR #45136)
#45136

This PR appears to address the same or very similar issue—capping session shell output—and was likely opened around the same time or addressing the same #45099 issue. You should check if PR #45136 is already merged or if it's an alternative approach to the same problem.