Snapshot runner workspace outside artifact topology

[chubes4]

Problem

The real executor-starting workspace E2E reaches RecipeArtifactsMountConflictError before Playground boot. execute-native-agent-task.mjs uses the host checkout as the /workspace seed while .codebox/agent-task-artifacts is nested inside that checkout. Recipe validation correctly rejects the recursive artifact/mount topology even though .codebox/** is excluded.

Discovered while validating #1786 after merged PR #1787.

Required outcome

• Create a filtered immutable seed snapshot outside the host checkout and artifact root before invoking the real CLI.
• Copy only allowed regular source files/directories; reject symlink escapes/special files; apply explicit excludes during copy.
• Use the external snapshot as canonical task_input.workspaces[].seed.source; recipe creates its own disposable /workspace and baseline.
• Clean snapshot after runtime while preserving normalized provenance.
• Full execute-script + real CLI + Playground intercepted-provider E2E reads/edits and applies a host patch.
• Preserve failure evidence and source privacy.
• Release with merged Propagate runner workspaces into agent tasks #1787 and rerun.