refactor: move V1 code into v1/ subdirectory

- git mv cmd/ internal/ pkg/ test/ go.mod go.sum Makefile build.sh docs/ v1/
- Create v1/README.md with V1 documentation
- Update root README for V1 + V2 structure
- V1 still builds (cd v1 && go build ./cmd/hostkeeper) and 105 tests pass
- Root is now clean for V2 development

v1/RELEASE_CHECKLIST.md

@@ -0,0 +1,41 @@
+# Release Checklist
+
+## Pre-Release
+
+- [ ] All tests pass: `make test`
+- [ ] Test coverage is adequate: `make test-coverage`
+- [ ] `go vet` passes: `make vet`
+- [ ] Lint passes: `make lint`
+- [ ] All platforms build successfully: `./build.sh`
+- [ ] `CHANGELOG.md` is up to date
+- [ ] Version is updated in source
+- [ ] Documentation is current
+
+## Testing
+
+- [ ] Integration tests pass: `go test ./test/integration/`
+- [ ] Manual smoke test all commands:
+  - [ ] `hostkeeper add`
+  - [ ] `hostkeeper list`
+  - [ ] `hostkeeper connect`
+  - [ ] `hostkeeper edit`
+  - [ ] `hostkeeper delete`
+  - [ ] `hostkeeper export`
+  - [ ] `hostkeeper import`
+  - [ ] `hostkeeper tui`
+  - [ ] `hostkeeper version`
+  - [ ] `hostkeeper completion`
+
+## Release
+
+- [ ] Tag the release: `git tag v1.0.0`
+- [ ] Push tags: `git push origin --tags`
+- [ ] Build release binaries: `./build.sh`
+- [ ] Verify checksums in `build/checksums.txt`
+- [ ] Create GitHub release with binaries attached
+- [ ] Post-release announcement (if applicable)
+
+## Post-Release
+
+- [ ] Update PROJECT_STATE.md
+- [ ] Start next milestone planning