§22 S2: collection-scoped list/get/propose endpoints

Refactor the project-scoped serve/propose internals into collection-grained
helpers (_list_rfcs_for_collection, _get_rfc_for_collection,
_propose_into_collection); add routes under
/api/projects/<id>/collections/<cid>/rfcs[/<slug>|/propose]. Propose writes
the entry under the target collection's <subfolder>/rfcs via a new rfcs_dir
param on bot.open_idea_pr. Default-collection routes preserved as wrappers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/bot.py

@@ -175,12 +175,16 @@ class Bot:
         file_contents: str,
         pr_title: str,
         pr_description: str,
+        rfcs_dir: str = "rfcs",
     ) -> dict:
-        """Per §9.1: open a meta-repo PR adding one file under rfcs/.
+        """Per §9.1: open a meta-repo PR adding one file under `<rfcs_dir>/`.
 
         One file per PR keeps idea submissions atomic and conflict-free.
         The PR title and the file-add commit subject share §9.2's fixed
-        pattern; callers compose `pr_title` as `Propose: <Title>`.
+        pattern; callers compose `pr_title` as `Propose: <Title>`. §22 S2:
+        `rfcs_dir` carries the target collection's `<subfolder>/rfcs` so a
+        propose into a named collection writes under its subfolder; it
+        defaults to `rfcs` (the default collection / shipped behaviour).
         """
         branch = f"propose/{slug}"
         await self._gitea.create_branch(org, meta_repo, branch, from_branch="main")
@@ -189,7 +193,7 @@ class Bot:
         created = await self._gitea.create_file(
             org,
             meta_repo,
-            f"rfcs/{slug}.md",
+            f"{rfcs_dir}/{slug}.md",
             content=file_contents,
             message=commit_message,
             branch=branch,