v0.15.6

0.15.6 tightens daemon monitor idle output and adds architecture records for
the compiled-plan and workspace-baseline decisions that now govern the runtime.

Added

• Added ADR-0005 for compiled_plan.json as the runtime-authoritative graph
  plan.
• Added ADR-0006 for explicit workspace baselines and managed upgrade
  behavior.

Changed

• Throttled repeated runtime_idle reason=no_work basic monitor lines into a
  120-second heartbeat while preserving the first idle line and resetting after
  runtime activity or a different idle reason.
• Expanded CLI reference coverage for every millrace skills subcommand and
  its core options.
• Updated the operator manual with compile show, queue inspection, modes,
  skills commands, and basic-monitor idle-heartbeat guidance.