feat(cli): unify plan and task command grammar

plan and task now share the same noun-verb shape. plan grows the human
subcommands it was missing; task list and task show use the same record
and table renderers as the rest of the CLI.

plan subcommands:
- plan list           list all plans, marking the active one with *
- plan show <id>      compact record with phases and task table
- plan current        show the active plan
- plan set-active <id> switch the active plan
- plan delete <id>    delete a plan and its tasks (with confirmation)
- plan tool --params <json>  the JSON entry used by /taskwing:plan

task command changes:
- task list           drops the 🤖 panel; uses ┌─ Tasks ─ header and the shared Table renderer
- task show           renders the task as a key:value Record (same shape as task next)

Slash command update:
- skills/taskwing-plan/SKILL.md now calls `taskwing plan tool --params ...`

Author: josephgoksu