Add onboarding to connect repos without remotes

doc/architecture.md

@@ -9,11 +9,15 @@
 - `git-tui`:
   - Initializes and runs the curses application loop.
   - Handles startup bootstrap for non-repository directories.
+  - Handles startup onboarding for repositories without configured remotes.
   - Renders a menu-driven UI with keyboard navigation.
   - Executes Git subprocess commands and displays command output.
   - Executes provider-specific publication for new remote repositories:
     - GitHub via `gh` CLI.
     - Gitea via HTTPS API and token-authenticated initial push.
+  - Executes provider-specific connection to existing remote repositories:
+    - GitHub by resolving owner or URL input to `origin`.
+    - Gitea by resolving owner and base URL or URL input to `origin`.
   - Implements file selection for staged and unstaged operations.
 
 ## Public API and Contracts

doc/usage.md

@@ -44,6 +44,19 @@
   - `GITEA_TOKEN` or `GITEA_ACCESS_TOKEN` for API token default.
 - Git credential prompts are disabled inside `git-tui`; authentication issues are reported in the UI instead of opening an interactive terminal prompt.
 
+## Existing Repository Without Remote Flow
+
+- If the current directory is already a Git repository and has no remotes configured, `git-tui` offers startup actions:
+  - Connect to an existing remote repository.
+  - Continue without remote configuration.
+  - Quit.
+- Connect existing remote flow:
+  - Chooses provider: GitHub or Gitea.
+  - GitHub accepts `owner/name` or full clone URL input.
+  - Gitea accepts `owner/name` or full clone URL input, and uses `GITEA_URL` or `GITEA_BASE_URL` defaults when a base URL is needed.
+  - Configures remote `origin` to the selected repository URL.
+  - Optionally pushes current branch with upstream tracking (`git push -u origin <branch>`).
+
 ## Key Commands in UI
 
 - Main menu: