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>

contrib/claude/agent.md

@@ -72,6 +72,27 @@ shutdown broadcast onto a per-run `cancelRun(agent.ErrSuspendForCheckpoint)`,
 so the same contract drives both the public Go API and the worker
 infrastructure path.
 
+## Prompt templates
+
+Prompt files with placeholders use `.txt.tmpl` (see general template naming
+convention in `.cursor/rules/template-files.mdc`).
+
+Use `agent.WithInstructionsFunc` to build prompts dynamically at runtime:
+
+```go
+//go:embed prompts/tracker_identification.txt.tmpl
+var trackerIdentificationPrompt string
+
+func trackerMappingInstructions(_ context.Context, _ *agent.Agent) string {
+    categories := coredata.ThirdPartyCategories()
+    parts := make([]string, len(categories))
+    for i, c := range categories {
+        parts[i] = string(c)
+    }
+    return strings.Replace(trackerIdentificationPrompt, "{{.Categories}}", strings.Join(parts, ", "), 1)
+}
+```
+
 ## Limits
 
 - Max turns: 10 (default)