fix(compaction): use neutral text for compaction replay message

[leohenon]

What does this PR do?

Closes #13838

Compaction injects What did we do so far? as a user message. The model treats this as a real request and sometimes generates an unwanted summary.

Changed the text to [Compacted].

How did you verify your code works?

Updated test in message-v2.test.ts and verified manually.

[Copilot]

Pull request overview

This PR addresses an issue where compaction replay injected a synthetic user prompt (“What did we do so far?”) that could trigger unintended model summaries, replacing it with a neutral marker.

Changes:

• Replaced the compaction replay injected user text with "[Compacted]".
• Updated the corresponding unit test expectation in message-v2.test.ts.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/opencode/src/session/message-v2.ts	Updates the synthetic compaction text injected into user model messages to a neutral marker.
packages/opencode/test/session/message-v2.test.ts	Updates test expectations to match the new injected compaction marker text.

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