Add PATCH /agents/:id/instructions-path endpoint for setting or clearing
an agent's instructions file path. Supports relative path resolution
against adapterConfig.cwd. Enforce chain-of-command authorization —
only the agent itself, board members, or ancestor managers can update.
Guard instructions path keys in the general agent update endpoint too.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>