fix(TaskExecute): keep the delegation flow reachable

[LCorleone]

Fixes #40

Problem

As detailed in #40, orchestrator workflows almost never reach the TaskExecute delegation path. The mechanics (RPC handshake, spawn) work — the model gets steered away from them by three interacting prompt-level friction points:

1. The system-reminder pushes "continue on with the tasks at hand" (do it yourself), the opposite of delegation
2. TaskCreate's agentType is optional and easy to omit, and TaskExecute refuses tasks without it — the most common dead-end
3. TaskExecute's guideline "Never use the Agent tool for tasks launched via TaskExecute" gets over-generalized, blocking the fallback even after a refusal

Net effect: task flow entered → task created without agentType → refusal → no escape hatch → stuck loop.

Changes

All in src/index.ts, prompt-level only — no schema or behaviour change beyond the default:

• TaskCreate defaults agentType to "general-purpose" — a plain task is now delegable instead of dead-ending at TaskExecute. Only pre-existing tasks lack agentType now.
• TaskExecute's guideline re-scoped to what it was meant to prevent: spawning a duplicate Agent for a task already running under TaskExecute (use TaskOutput to check on it). The Agent-tool fallback stays open for refused/not-launched tasks.
• The no-agentType refusal (legacy tasks only) now points at the fallback: "…or run the work directly (e.g. via the Agent tool) since no subagent is running for it".
• The system-reminder is delegation-aware: the JSON echo includes agentType, and when any shown pending task has one, the closing line steers toward TaskExecute instead of "continue on with the tasks at hand". agentType goes through sanitizeField() like every other echoed string.

README updated where agentType is documented (CONTRIBUTING: user-facing changes). CHANGELOG untouched per CONTRIBUTING.

Tests

• new: reminder steers pending default-agentType tasks toward TaskExecute (and no longer contains the "continue on" line)
• new: tasks without agentType (stripped via metadata null-delete) keep the generic closing line
• reworked: "rejects tasks without agentType" → plain TaskCreate now spawns with general-purpose
• new: legacy no-agentType task (seeded directly in a store file) still refused, with the run-it-directly hint
• mixed valid/invalid batch updated for the default

npm run lint, typecheck, build clean; 407/407 tests pass.