chore(openspec): drop 9 superseded proposals + 11 stub archive files

Drop 9 batch proposals that are superseded by the boocode-lift-analysis
(boocontext-audit, conductor upgrades, self-healing/verify-gate skills):
add-3tier-memory, import-llm-evaluator, import-pregel-engine, plugin-platform,
conductor-evolution, code-intelligence-upgrade, dev-workflow, ui-overhaul,
agent-reliability.

Delete 11 stub archive files (49-66B each, 'Status: Shipped. Archived.' only)
that provide zero documentation value over the existing CHANGELOG.md + git tags.

openspec/changes/archived/2026-06-07-eval-sandbox-agent-runtime/specs/sandbox-snapshots/spec.md

@@ -0,0 +1,59 @@
+## ADDED Requirements
+
+### Requirement: Snapshot creation
+
+The system SHALL provide `sandbox.snapshot()` to create a point-in-time filesystem snapshot.
+
+Parameters:
+- `expiration?: number` — TTL in milliseconds (0 for no expiration)
+
+#### Scenario: Snapshot stops the session and returns Snapshot instance
+
+- **WHEN** `sandbox.snapshot()` is called on a running sandbox
+- **THEN** the current session SHALL be stopped and a `Snapshot` SHALL be returned
+
+### Requirement: Snapshot retrieval and listing
+
+The system SHALL provide `Snapshot.get()`, `Snapshot.list()`, and `Snapshot.tree()` for managing snapshots.
+
+#### Scenario: Retrieve snapshot by ID
+
+- **WHEN** `Snapshot.get({ snapshotId: "snap_abc" })` is called
+- **THEN** it SHALL return the snapshot with matching ID
+
+#### Scenario: List snapshots with pagination
+
+- **WHEN** `Snapshot.list({ name: "my-sandbox" })` is called
+- **THEN** it SHALL return a paginated list of snapshots for that sandbox
+
+#### Scenario: Ancestry tree is accessible
+
+- **WHEN** `Snapshot.tree({ snapshotId: "snap_abc" })` is called
+- **THEN** it SHALL return the ancestry tree of the snapshot
+
+### Requirement: Snapshot deletion
+
+The system SHALL provide `snapshot.delete()` to remove a snapshot.
+
+#### Scenario: Deleted snapshot is no longer listable
+
+- **WHEN** `snapshot.delete()` is called and then `Snapshot.list()` is called
+- **THEN** the deleted snapshot SHALL no longer appear in the list
+
+### Requirement: Snapshot-based sandbox creation
+
+The system SHALL support creating sandboxes from snapshots via `Sandbox.create({ source: { type: "snapshot", snapshotId } })`.
+
+#### Scenario: Sandbox created from snapshot has matching filesystem
+
+- **WHEN** a sandbox is created with a snapshot source and a file is written, then another sandbox is created from the resulting snapshot
+- **THEN** the second sandbox SHALL contain the file from the first
+
+### Requirement: Snapshot retention
+
+The system SHALL support `keepLastSnapshots` retention policy on sandboxes.
+
+#### Scenario: Retention evicts oldest snapshots
+
+- **WHEN** a sandbox has `keepLastSnapshots: { count: 3 }` and a 4th snapshot is created
+- **THEN** the oldest snapshot SHALL be evicted