fix: make agent prompts imperative about initiative delegation workflow

[danielkov]

Summary

• Agents planning multi-project initiatives were creating tasks directly instead of delegating to sub-agents
• Redesigned all agent-facing prompts to front-load constraints and distinguish single-project vs multi-project paths
• granary initiate now opens with a CRITICAL: Delegation-Only Workflow section that explicitly forbids direct task creation
• granary plan (without --project) now includes a scope guard warning initiative agents to stop and use initiate instead
• Entrypoint and --help now present a decision tree ("Choose ONE entry point based on scope") instead of a flat command list

New granary initiate prompt (top section)

## CRITICAL: Delegation-Only Workflow

You are the initiative coordinator. Your job is to create projects and delegate.
Do NOT create tasks directly. Do NOT use `granary project <id> tasks create`.
Task creation is handled by sub-agents via `granary plan --project <id>`.

## Workflow

1. Create projects with descriptions (Step 1 below)
2. Set inter-project dependencies (Step 2 below)
3. Delegate planning to sub-agents: `granary plan --project <id>` for each project
4. Verify the initiative

New granary plan scope guard

## Scope: Single-Project Planning

This workflow is for planning ONE project with tasks.
If this is part of a multi-project initiative, stop here.
Use `granary initiate "Initiative name"` instead — it guides you through
creating projects and delegating task planning to sub-agents.

New entrypoint prompt

Available granary workflows:

Choose ONE entry point based on scope:
- Single project (one feature/fix): `granary plan "Feature name"`
- Multiple projects (cross-cutting work): `granary initiate "Initiative name"`

Other commands:
- Work on task: `granary work <task-id>`
- Search: `granary search "keyword"`

New --help text

AGENTS (AI/LLM):
  Choose ONE entry point based on scope:

  Single project (one feature/fix):
    granary plan "Feature name"

  Multiple projects (cross-cutting work):
    granary initiate "Initiative name"
    NOTE: Do NOT use `granary plan` for multi-project work.
    `initiate` guides you through creating projects and delegating task planning to sub-agents.

✻ Clauded...