fix: include first message in condense summarization

[roomote]

Description

This PR fixes issue #8293 where the condense feature was omitting the initial user message from the summarization process, causing the task context to be lost when resuming after condensation.

Problem

When condensing a conversation:

• The first user message was preserved in the final result but excluded from what gets summarized
• This caused the summary to miss the initial ask/context
• When resuming, the agent would re-answer the original question instead of continuing the work

Solution

Modified the summarizeConversation function to include the first message in the content that gets summarized:

• Changed from messages.slice(1, -N_MESSAGES_TO_KEEP) to messages.slice(0, -N_MESSAGES_TO_KEEP)
• The first message is now both included in the summary AND preserved separately as before

Testing

• Added a new test case that verifies the first message is included in the summarization content
• All existing tests continue to pass (33 tests total across both test files)
• The fix maintains backward compatibility with existing behavior

Impact

This ensures that:

• The initial task context is captured in the summary
• Resume operations continue from where they left off instead of re-answering the original question
• Both manual and automatic condense operations preserve full context

Fixes #8293

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Important

Fixes issue #8293 by including the first user message in the summarization process to preserve initial task context in summarizeConversation.

• Behavior:
  • Fixes issue [BUG] Condense omits initial ask; summary loses context, resume re-answers ask #8293 by including the first user message in the summarization process in summarizeConversation.
  • Changes messages.slice(1, -N_MESSAGES_TO_KEEP) to messages.slice(0, -N_MESSAGES_TO_KEEP) in index.ts.
  • Ensures initial task context is captured in the summary and preserved separately.
• Testing:
  • Adds a test case in condense.spec.ts to verify inclusion of the first message in summarization.
  • All existing tests pass, ensuring backward compatibility.
• Impact:
  • Preserves full context for resume operations, preventing re-answering of original questions.
  • Supports both manual and automatic condense operations.

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[roomote]

Self-review initiated: auditing my own code with the impartiality of a mirror judging a mirror.

[roomote]

P2: If N_MESSAGES_TO_KEEP === 0, slice(0, -0) yields an empty array, causing no messages to be summarized. Consider normalizing the end index to undefined when N_MESSAGES_TO_KEEP is 0 (e.g., const end = N_MESSAGES_TO_KEEP ? -N_MESSAGES_TO_KEEP : undefined; const messagesToSummarize = getMessagesSinceLastSummary(messages.slice(0, end));).

[roomote]

P2: This test override uses Anthropic.Messages.MessageParam[], coupling the test to a specific provider SDK. Prefer a provider-agnostic message type to reduce brittleness across providers.