Unify skill/prompt seeding with markdown import pipeline #1056

Overview

Skill and prompt seed data is currently defined as Rust structs (SeedSkill, SeedPrompt, SeedGuidancePrompt) with manual to_node() conversions and custom parent-child creation logic in db.rs. This duplicates the hierarchy creation that create_nodes_from_markdown already handles well.

Problem

• Two separate code paths for creating root nodes with children: markdown import and programmatic seeding
• Adding a new skill requires Rust code changes and recompilation
• The markdown import already handles hierarchy, ordering, and batch creation efficiently
• Skills are structurally identical to markdown documents: a root node with child content nodes

Proposed Solution

Extend create_nodes_from_markdown (or add a parallel create_nodes_from_template) to support:

1. Root node type override — caller specifies node_type: "skill" instead of inferring from markdown syntax
2. Root properties injection — caller passes description, tool_whitelist, max_iterations etc.
3. Child type override — children created as prompt nodes with priority/source/template_syntax instead of text/header

This would allow seed skills to be defined as markdown files (or embedded strings):

# Schema Creation

To create a new schema, use create_node with node_type="schema".
Fields are defined in properties.fields as an array.

## Example Structure
{json example}

With metadata passed alongside:

{
  "markdown_content": "...",
  "root_node_type": "skill",
  "root_properties": {"description": "...", "tool_whitelist": ["create_node"]},
  "child_node_type": "prompt",
  "child_properties": {"source": "built-in", "template_syntax": "plain"}
}

Benefits

• Single code path for hierarchy creation
• Skills definable as markdown (no Rust changes for new skills)
• Future: users create skills by writing markdown in the UI
• Eliminates SeedSkill, SeedGuidancePrompt structs and custom seeding logic

Current State

• SeedSkill + SeedGuidancePrompt in packages/agent/src/skill_pipeline.rs
• SeedPrompt in packages/agent/src/prompt_assembler.rs
• Custom seeding in packages/desktop-app/src-tauri/src/commands/db.rs
• Markdown import in packages/core/src/mcp/handlers/markdown.rs

Dependencies

• Prompt node type with PromptAssembler service and graph-override layering #1049, Skill node type with intent-driven push-based injection and tool whitelisting #1050, Semantic skill discovery via find_skills tool for 8B model and external agents #1051 (merged — prompt/skill node infrastructure)