Collapse history into Workspace

[zac]

Summary

• Collapse the separate History/session model into the Workspace actor as the source of truth for filesystem state, mutation logging, checkpoints, rollback, branch, and merge.
• Add top-level public checkpoint/event types while keeping mutation records and checkpoint stores internal.
• Rewrite checkpoint, store, snapshot, text diff, and internals tests around the new Workspace surface, including branch isolation, merge conflict, polling, symlink delta, and binary write coverage.
• Rewrite README to describe the branch/merge model and storage caveats.

Validation

• swift test passes: 115 tests in 9 suites.