Build tracker identification prompt categories dynamically

Generate the category list from coredata.ThirdPartyCategories() at
runtime instead of hardcoding it in the prompt text. Type the
TrackerIdentification.Category field as coredata.ThirdPartyCategory so
JSON unmarshaling validates values automatically.

Also documents the .txt.tmpl template file naming convention.

Signed-off-by: Émile Ré <emile@probo.com>

AGENTS.md

@@ -25,6 +25,7 @@ Detailed guides for specific subsystems live in `contrib/claude/`:
 - [`contrib/claude/react-components.md`](contrib/claude/react-components.md) — React component shape (file/export, props, configure vs data via hooks)
 - [`contrib/claude/ui.md`](contrib/claude/ui.md) — @probo/ui, Tailwind, tailwind-variants, folders, skeletons, compound components
 - [`contrib/claude/config.md`](contrib/claude/config.md) — Configuration propagation (all files to update when config changes)
+- [`contrib/claude/file-naming.md`](contrib/claude/file-naming.md) — File naming conventions (template files, extensions)
 - [`contrib/claude/commit.md`](contrib/claude/commit.md) — Commit message conventions
 - [`contrib/claude/license.md`](contrib/claude/license.md) — ISC license header (all file types)
 - [`contrib/claude/release.md`](contrib/claude/release.md) — Release process (version bump, changelog, tag, push)