refactor: use PI_CODING_AGENT_DIR env var and remove unused subagent infrastructure

[kostyay]

• Replace hardcoded ~/.pi/agent paths with PI_CODING_AGENT_DIR environment variable throughout extensions, with fallback to $HOME/.pi/agent when unset
• Update control socket path from ~/.pi/session-control to $PI_CODING_AGENT_DIR/../session-control for consistent storage hierarchy
• Update stash, sandbox, and session-breakdown storage paths to use PI_CODING_AGENT_DIR instead of hardcoded home directory paths
• Remove unused subagent infrastructure: agent definitions (planner, researcher, reviewer, scout, worker), cmux session management, and associated test suites (~2,200 lines)
• Simplify brainstorm skill documentation to focus on ask_question tool patterns, removing batched question guidance and redundant examples
• Remove closed ticket references (.tickets/as-0673.md, .tickets/as-5a68.md) and update CHANGELOG with refactor summary