feat: merge oo-cli and extend library with Calendar/CRM/subtasks/files

- Add Calendar helpers (ListCalendars/ListEvents/AddEvent/DeleteEvent)
- Add CRM helpers: contacts, opportunities, cases, CRM tasks, history notes
- Add project-task extras (ListTasks/ListAllTasks/GetTaskByID/AddTask/
  AddSubtask/UpdateTaskStatus/DeleteTask, GetProjectByID)
- Add UploadOpportunityFile (multipart) and SelfUserID caching
- Introduce Defaults struct, SetDefaults, GetEnvironmentDefaults;
  GetEnvironmentCredentials now accepts ONLYOFFICE_HOST/NAME/PASSWORD aliases
- Add public Authenticate() helper that eagerly primes the token
- Bundle Cobra CLI at cmd/oo-cli with full command surface
  (calendar, tasks, subtasks, CRM, applications-sync); CLI-only deps
  (cobra, godotenv) kept out of the library surface
- Add httptest-based unit tests for form/multipart helpers
- Docs: CHANGELOG, AGENTS, .env.example, README updates for v0.3 features

Consolidates the standalone eSlider/oo-cli tool into this canonical Go
module (see eSlider/inventar ASR-0008).

Made-with: Cursor

.env.example

@@ -0,0 +1,16 @@
+# OnlyOffice Workspace credentials. Used by cmd/oo-cli (the library itself
+# does not load .env files — only its CLI does, via godotenv).
+#
+# Primary names:
+ONLYOFFICE_URL=https://office.example.com
+ONLYOFFICE_USER=
+ONLYOFFICE_PASS=
+
+# Accepted aliases (only one of each pair is needed):
+# ONLYOFFICE_HOST=
+# ONLYOFFICE_NAME=
+# ONLYOFFICE_PASSWORD=
+
+# Optional defaults for calendar/project helpers:
+ONLYOFFICE_CALENDAR_ID=1
+ONLYOFFICE_PROJECT_ID=33