feat(files): project/task Documents API + oo projects|tasks files

Library (files.go):
- Types: FileEntry, FolderEntry, ProjectFilesResponse.
- GetProjectFiles, GetTaskFiles, GetFile, UploadProjectFile (POST .../upload),
  AttachFilesToTask (POST form files=), UploadTaskFile (upload+attach),
  DetachTaskFile (DELETE ?fileid=), RenameFile, DeleteFiles, DownloadFile.
- Helpers: FileEntryNumericID, FileEntryTitle, SafeLocalFileName, projectIDFromTaskMap.

http.go: putJSON for JSON PUT bodies.

CLI: oo projects files {list,upload,download,rename,delete}, oo tasks files
{list,upload,detach}; projects_files.go + tasks_files.go.

Tests: files_integration_test.go (live), files_test.go + testdata (decode).

Docs: CHANGELOG 0.6.0, README, AGENTS.md, cmd/oo/main.go tree.
Made-with: Cursor

README.md

@@ -559,13 +559,23 @@ oo opportunities stages
 oo cases list
 oo crm-tasks categories
 oo applications sync --path ./applications/2026 --apply
+
+# Project Documents (files module)
+oo projects files list 33
+oo projects files upload 33 ./notes.md
+oo projects files download 12345 --to ./copy.md
+oo projects files rename 12345 notes-v2.md
+oo projects files delete 12345
+oo tasks files list 208
+oo tasks files upload 208 ./cv.pdf
+oo tasks files detach 208 12345
 ```
 
 | Subject | Verbs |
 |---|---|
 | `calendar` | `list`, `events`, `add`, `delete` |
-| `projects` | `list`, `get`, `milestones`, `create`, `update`, `delete` |
-| `tasks` | `list`, `get`, `create`, `update`, `delete`, `subtask add` |
+| `projects` | `list`, `get`, `milestones`, `create`, `update`, `delete`, **`files`** (`list`, `upload`, `download`, `rename`, `delete`) |
+| `tasks` | `list`, `get`, `create`, `update`, `delete`, `subtask add`, **`files`** (`list`, `upload`, `detach`) |
 | `users` | `list`, `self` (alias: `oo whoami`) |
 | `contacts` | `list`, `get`, `delete`, `info-add` |
 | `persons` | `list` (filtered), `create`, `delete` |