fix: preserve live context during memory compression

[vaibhavmashal]

Problem

Three-zone compression (internal/llmloop/compression.go) discards live context it is designed to preserve, in three related ways:

1. When every round fits the token budget, the "everything fits" branch marks the whole tail as the compress zone, causing every round to be summarized.
2. The frozen zone's tokens are never reserved from the budget, allowing the active zone to overfill past the warning threshold.
3. A missing compression template truncates the conversation to 2 messages instead of leaving it alone.

This leads to unintended truncation and summarization of recent context.

Solution

This PR applies the following fixes:

• Represents the empty compress zone as compressEnd == frozenEnd, rather than compressEnd == len(messages).
• Reserves the token count of messages[:frozenEnd] in addition to prevSummaryTokenEstimate when computing the active zone size.
• Makes runCompression return the conversation unchanged when no template is configured instead of slicing it down to the first 2 messages.

Changes Made

• internal/llmloop/compression.go:
  • Modified partitionMessages to compute and pass the frozen zone token count to computeActiveZoneSize.
  • Modified partitionMessages to set compressEnd = result.frozenEnd instead of len(messages) when everything fits in the active zone.
  • Modified runCompression to return msgs instead of msgs[:min(len(msgs), 2)] when the compression template is empty.
• internal/llmloop/compression_zone_repro_test.go: Added reproduction tests to ensure these edge cases are covered.

Testing

• Added automated unit tests to verify each edge case: TestRepro_EverythingFitsSummarizesLiveTail, TestRepro_FrozenZoneNotReservedFromBudget, and TestRepro_MissingTemplateTruncatesConversation.

Fixes #838

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