feat(copilot): implement TranscriptAnalyzer for JSONL transcripts

[peyton-alt]

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[cursor]

PR Summary

Medium Risk
Introduces new transcript-parsing logic that influences checkpoint metadata (modified files/prompts/summary); mistakes in event/schema handling could lead to incomplete or incorrect checkpoints.

Overview
Copilot CLI sessions now implement agent.TranscriptAnalyzer, enabling checkpoint metadata to be derived directly from Copilot’s events.jsonl transcript.

This adds JSONL parsing to compute transcript position (line count), extract modified files from tool.execution_complete events (via toolTelemetry.properties.filePaths), collect user prompts from user.message, and use the last assistant.message as a session summary, with comprehensive unit tests and updated Copilot integration docs noting the new event usage and lack of token metrics.

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[Copilot]

Pull request overview

Implements Copilot CLI transcript analysis for JSONL events.jsonl so Entire can extract prompts, summaries, and modified-file metadata from Copilot sessions.

Changes:

• Added TranscriptAnalyzer implementation for CopilotCLIAgent, including prompt/summary extraction and modified-file detection via tool.execution_complete events.
• Added unit tests covering event parsing, prompt/summary extraction, modified-file extraction, and transcript position counting.
• Updated Copilot CLI agent documentation to describe the new transcript-derived metadata behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
cmd/entire/cli/agent/copilotcli/transcript.go	Adds JSONL parsing and TranscriptAnalyzer methods for Copilot CLI transcripts.
cmd/entire/cli/agent/copilotcli/transcript_test.go	Adds tests for parsing and extraction logic, plus interface implementation checks.
cmd/entire/cli/agent/copilotcli/AGENT.md	Documents tool.execution_complete and the new TranscriptAnalyzer behavior.