Add metadata-driven llms.txt and extract starter templates

[dannote]

Summary

• Add Astral.Plugin.LLMs, generating only /llms.txt from route and page metadata in builds and development.
• Discover eligible HTML pages and generated routes by default, respecting output precedence and excluding drafts, error pages, metadata opt-outs, and explicit exclusions.
• Support ordered curated sections, collection selectors, inferred link metadata, overrides, and external links.
• Serialize Markdown directly through MDEx's AST. No page-body conversion, Markdown alternates, content dumps, or additional dependencies.
• Move starter files into packaged EEx templates consumed through Igniter; enable the index with the project title.
• Reference Volt's shared browser declarations instead of generating local ImportMeta definitions.

Validation

• mix ci: 232 tests passed; formatting, compilation, JavaScript checks, Credo, Dialyzer, duplication, and architecture checks passed.
• Tests cover generated output, development responses, metadata inference, exclusions, selection, output precedence, escaping, URL prefixes, and invalid configuration.
• Fresh starter fixture type-checked with TypeScript 7.0.2 against Volt's shared declarations.
• Generated a candidate index from dannote.net's actual metadata without changing the blog's published dependency.

Dependencies

The starter picks up Volt's shared CSS declarations through its existing deps/volt/priv/types/client/**/*.d.ts glob, so no dependency floor change is required. Volt 0.18.1 satisfies the existing ~> 0.18.0 requirement.

No version bump or publication is included in this PR.