feat(projects): add project registry (Plane id -> repo/prefix mapping)

ORCH-6: src/projects.py introduces ProjectConfig + resolvers
(get_project_by_plane_id/by_repo, known_plane_project_ids) keyed by
Plane project uuid. Source: ORCH_PROJECTS_JSON env (config.projects_json),
with a built-in default registry (enduro-trails + orchestrator) and
robust parsing (malformed JSON/entries fall back to default).

src/config.py

@@ -16,6 +16,11 @@ class Settings(BaseSettings):
     gitea_owner: str = "admin"
     default_repo: str = "enduro-trails"
 
+    # ORCH-6: multi-repo project registry. JSON array of
+    #   {plane_project_id, repo, work_item_prefix, name}.
+    # Empty -> built-in default registry in src/projects.py.
+    projects_json: str = ""
+
     # Claude CLI
     claude_bin: str = "/opt/claude-code/bin/claude.exe"
     repos_dir: str = "/repos"