Use shared native history without writing duplicate checkpoints

[Brian Krabach (bkrabach)]

Session resume currently prefers a separate shared checkpoint, which can hide newer transcript writes from older CLI versions. This change makes the existing transcript.jsonl and metadata.json authoritative again through Foundation's shared SessionHistoryStore, while retaining cross-host writer locks. Saving does not create another checkpoint; checkpoint-only historical sessions remain readable.

Context Intelligence activity is read from its existing capture, including configured relocation, to restore prior session cost. The legacy root event log remains a fallback when CI capture is absent. Forks retain existing legacy-log behavior but do not copy or rewrite CI evidence. Session naming, provider continuation data, reminder flags, unknown metadata, backup recovery, and config snapshots keep their native format. Unrecoverable canonical corruption now fails visibly rather than returning an empty history.

Depends on microsoft/amplifier-foundation#396; the dependency is pinned to its reviewed implementation commit and should be refreshed to the merged commit before merging this PR. No Context Intelligence bundle change, migration, or new storage format is required. Transcript-only CLI versions can read these saves; versions that prefer shared checkpoints must also adopt this change to stop selecting stale checkpoints.

Validation:

• Full default suite: 2,358 passed, 45 skipped, 13 integration tests deselected, 1 expected failure. The separate POSIX integration run also passed all 13 tests.
• Disposable installed-old-CLI → updated web → installed-old-CLI and updated-CLI → installed-old-CLI round trips preserve messages, provider metadata, reminders, attachments, names, and untouched CI event bytes; no checkpoint is created.
• Focused regressions cover native precedence over stale checkpoints, valid empty transcripts, backups, corrupt files, ownership, and CI cost scoping/relocation.