gnd: Add CLI documentation and migration guide

- Add comprehensive README.md with command reference for all gnd commands
- Add MIGRATING_FROM_GRAPH_CLI.md guide for users transitioning from graph-cli
- Document all flags, examples, and common workflows
- Mark documentation items as complete in the plan

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: lutter

docs/plans/gnd-cli-expansion.md

@@ -285,9 +285,9 @@ gnd/src/
 - [x] Add comprehensive tests for prompt module (network completer, source type)
 - [x] Add comprehensive tests for init command (interactive mode detection)
 - [ ] Ensure all edge cases are covered (ongoing)
-- [ ] Documentation:
-  - [ ] CLI usage docs (README with command reference, examples, common workflows)
-  - [ ] Migration guide from graph-cli to gnd (differences, compatibility notes)
+- [x] Documentation:
+  - [x] CLI usage docs (README with command reference, examples, common workflows)
+  - [x] Migration guide from graph-cli to gnd (differences, compatibility notes)
 - [x] Shell completions (bash, elvish, fish, powershell, zsh via clap_complete)
 
 ## Key Decisions