Add Codex support for explain --generate

[peyton-alt]

Adds support to generate summary from codex session with entire explain -c <checkpointID> --generate

Avoids error failed to generate summary: transcript has no content to summarize

Note

Medium Risk
Adds a new parsing/scoping path for Codex transcripts used by explain --generate, which could affect checkpoint transcript slicing and summary accuracy for Codex sessions but is localized to transcript processing.

Overview
explain --generate and transcript rendering now support Codex sessions by scoping checkpoint transcripts using Codex response_item offsets (via new codex.SliceFromResponseItem) instead of generic line slicing.

summarize.BuildCondensedTranscriptFromBytes gains a Codex path that compacts Codex rollout JSONL into the shared condensed transcript format (including tool calls), and compact Codex format detection is broadened to recognize additional Codex line types. New tests cover Codex scoping and condensed-transcript extraction, and tool-detail extraction is hardened against nil inputs.

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[Copilot]

Pull request overview

Adds Codex rollout transcript support to the explain --generate summarization flow by enabling checkpoint-scoped parsing/compaction and condensed transcript extraction.

Changes:

• Expanded Codex format detection to recognize checkpoint-scoped Codex JSONL that may not include session_meta.
• Added Codex handling in summarization condensation by compacting Codex rollouts into the shared compact transcript schema and extracting user/assistant/tool entries.
• Updated transcript scoping for Codex in explain and added unit tests for Codex summarization/scoping.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
cmd/entire/cli/transcript/compact/codex.go	Broadens Codex JSONL detection to handle mid-file (scoped) transcripts.
cmd/entire/cli/summarize/summarize.go	Adds Codex-specific condensed transcript building via compact.Compact and parses compact JSONL into summary entries.
cmd/entire/cli/summarize/summarize_test.go	Adds a unit test validating Codex condensed transcript extraction.
cmd/entire/cli/explain.go	Scopes Codex transcripts using response_item-based slicing before generating summaries/intents.
cmd/entire/cli/explain_test.go	Adds a unit test for Codex checkpoint transcript scoping behavior.
cmd/entire/cli/agent/codex/transcript.go	Introduces SliceFromResponseItem helper used for Codex transcript scoping.