Introduce a persistent execution history system that records each
executable run (ref, timestamp, duration, status) and surfaces it
through enhanced flow logs commands. This replaces the previous
in-memory store with a new pkg/store backed by bbolt, moves cache
logic into pkg/cache for broader reuse, and adds a flow logs clear
subcommand for cleanup.

Key changes:

• New pkg/store with bbolt-backed data store for execution records
• Enhanced log viewer with filterable execution history (by verb,
  workspace, status, and time range)
• Parallel/serial runners now record execution results automatically
• Relocated cache package from internal/ to pkg/ for wider access
• Added execution history guide and updated CLI docs
• Added e2e tests for new log commands