Externalize pi-backtask into standalone plugin submodule

[artiombell]

Summary\n- move from local into standalone repo \n- add git submodule pinned to the standalone plugin\n- rewire recipe to load plugin from submodule\n- update recipe to accept explicit paths for external plugins\n- update README docs and project structure for submodule usage\n\n## Verification\n- Available recipes:

all                     # Open every extension in its own terminal window
default
ext-agent-chain         # 13. Agent chain: sequential pipeline orchestrator
ext-agent-team          # 10. Agent team: dispatcher orchestrator with team select and grid dashboard
ext-cross-agent         # 4. Cross-agent pi: load commands from .claude/, .gemini/, .codex/ dirs
ext-damage-control      # 12. Launch with Damage-Control safety auditing
ext-minimal             # 3. Minimal pi: model name + 10-block context meter
ext-pi-backtask         # 17. Pi backtask: background task runner + persistent task list (Ctrl+B/Ctrl+T)
ext-pi-pi               # 14. Pi Pi: meta-agent that builds Pi agents with parallel expert research
ext-pure-focus          # 2. Pure focus pi: strip footer and status line entirely
ext-purpose-gate        # 5. Purpose gate pi: declare intent before working, persistent widget, focus the system prompt on the ONE PURPOSE for this agent
ext-session-replay      # 15. Session Replay: scrollable timeline overlay of session history (legit)
ext-subagent-widget     # 8. Subagent widget: /sub <task> with live streaming progress
ext-system-select       # 11. System select: /system to pick an agent persona as system prompt
ext-theme-cycler        # 16. Theme cycler: Ctrl+X forward, Ctrl+Q backward, /theme picker
ext-tilldone            # 9. TillDone: task-driven discipline — define tasks before working
ext-tool-counter        # 6. Customized footer pi: Tool counter, model, branch, cwd, cost, etc.
ext-tool-counter-widget # 7. Tool counter widget: tool call counts in a below-editor widget
open +exts              # Accepts extension names (e.g. minimal) or explicit .ts paths (e.g. plugins/pi-backtask/pi-backtask.ts).
pi                      # 1. default pi\n-