refactor: call a checkpoint a checkpoint

[khaong]

while we're here - let's rename the branch to what it actually contains

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Note

Medium Risk
Branch-name changes affect where checkpoint metadata is written/read, so existing repos expecting entire/sessions/v1 may need migration or compatibility handling.

Overview
Renames the metadata orphan branch used to store committed checkpoint metadata from entire/sessions/v1 to entire/checkpoints/v1 by updating paths.MetadataBranchName.

Updates cleanup/shadow-branch logic, CLI tests, and helper scripts (test harness, attribution E2E, and migrate-sessions.sh) to reference the new branch name and ensure it remains excluded from shadow-branch cleanup.

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[Copilot]

Pull request overview

This PR renames the metadata branch from entire/sessions/v1 to entire/checkpoints/v1 to better reflect that the branch stores checkpoint metadata rather than session data. This is a simple constant rename with the goal of improving code clarity.

Changes:

• Updates the MetadataBranchName constant in paths/paths.go from "entire/sessions/v1" to "entire/checkpoints/v1"

[cursor]

Cursor Bugbot has reviewed your changes and found 3 potential issues.

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