chore: drop legacy contracts + supervisor task-grouping (#136)HEADmaster

The contracts table, supervisor task type, and all their backing
machinery have been inert for several PRs. The directives system reads
its own active contract body for spec text, and PR #135 removed the
last LLM surface that spawned supervisors.

This PR wipes the dead surface in one shot — the user authorised a DB
wipe, so the migration drops every legacy table with CASCADE rather
than carrying forward stub rows. Net change: −12k LOC across handlers,
repository, state, models, the TUI, and the listen module.

What's gone:
- contracts, contract_chat_*, contract_events, contract_repositories,
  contract_type_templates tables.
- supervisor_states, supervisor_heartbeats tables.
- mesh_chat_conversations, mesh_chat_messages tables.
- tasks.contract_id/is_supervisor/supervisor_task_id/supervisor_worktree_task_id columns.
- directive_steps.contract_id/contract_type columns.
- files.contract_id/contract_phase columns.
- history_events.contract_id/phase columns.
- The Contract/Supervisor/MeshChat handler + model + repository
  surface, plus the daemon TUI views that read them.
- The standalone listen.rs websocket handler (orphaned with the LLM).

What stays:
- mesh_supervisor handler: trimmed to just the questions + orders
  backchannel used by `makima directive ask` / `create-order` (kept
  the URL prefix for CLI client compat).
- directive_documents (the user-facing "contracts" surface).
- pending_questions in-memory state for the directive Ask flow.

cargo check, cargo test --lib (68 passed), tsc, and vite build all
clean.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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