feat(docs,rules): ship default content as editable markdown templates

[rgdevme]

Summary

• doc-rules.md is no longer fully rewritten on every agnos docs generate. It's now a user-editable file whose only managed region is a ```frontmatter fenced code block, re-injected from agnos.json#docs.metadata. Everything around the block is preserved.
• Default content for doc-rules.md, index.md, and content.md moves out of in-code template strings into real .md asset files under packages/domain-docs/templates/, so they can be edited as markdown.
• The rules domain follows the same pattern: the starter AGENTS.md content moves to packages/domain-rules/templates/agents.md, exposed via a new @luxia/domain-rules/template subpath export. core/init.ts now reads from it instead of an inline STARTER_RULES const — single source of truth.

How doc-rules.md regeneration works now

State	Behavior
File missing	Seed from templates/doc-rules.md, then inject current metadata into its ```frontmatter block
Block present	Only the block contents change when metadata differs; surrounding text untouched
Block deleted by user	Prepended back on next generate
Metadata unchanged	writeIfChanged no-ops

Metadata serialization runs through js-yaml so descriptions containing colons or quotes round-trip safely.

Architecture notes

• core now has a direct workspace dep on @luxia/domain-rules. To avoid a runtime cycle (the plugin's index.ts imports @luxia/core), the template loader lives in a separate src/template.ts module with no @luxia/core import, exported under the ./template subpath. pnpm reports the workspace cycle as a warning; it's inert at runtime.
• Build order matters once: @luxia/core must build before @luxia/domain-rules for DTS resolution. pnpm -r build handles this in topological order; in a clean clone, build core first if running packages in isolation.