feat: add conversation transcript lookup after summarization

[bhavyaus]

After conversation history is compacted, inform the model it can look up the full pre-compaction transcript via read_file.

[Copilot]

Pull request overview

This PR adds an experiment-gated “transcript lookup” capability so that after agent conversation history is compacted/summarized, the model can be told where to find the full pre-compaction transcript (and the read_file tool can be permitted to access that transcript location).

Changes:

• Introduces ConfigKey.ConversationTranscriptLookup and surfaces it in package.json/package.nls.json.
• Extends ISessionTranscriptService with isTranscriptUri() and implements it in the VS Code Node service.
• Allowlists transcript URIs for tool file reads and injects a transcript-path hint into the summary message when enabled.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/platform/configuration/common/configurationService.ts	Adds experiment-based config key for transcript lookup.
src/platform/chat/common/sessionTranscriptService.ts	Extends transcript service interface with isTranscriptUri.
src/extension/chat/vscode-node/sessionTranscriptService.ts	Implements isTranscriptUri by checking against the transcripts storage directory.
src/extension/tools/node/toolUtils.ts	Allows tool reads of transcript URIs (and treats them as non-external).
src/extension/prompts/node/agent/summarizedConversationHistory.tsx	Starts/flushes transcript session around summarization and appends transcript lookup hint to summary message.
package.json	Exposes the new setting in the extension configuration schema.
package.nls.json	Adds the localized description string for the new setting.

Comments suppressed due to low confidence (1)

src/extension/tools/node/toolUtils.ts:302

• isFileExternalAndNeedsConfirmation now treats transcript URIs as non-external, but there’s no test coverage for this new branch. Consider adding a unit test alongside the existing external-file confirmation tests to verify transcript paths return false (no confirmation) and don’t accidentally fall through to the external-file existence check.

	if (sessionTranscriptService.isTranscriptUri(normalizedUri)) {
		return false;
	}