feat: add validate-config subcommand

[ivyrhodes]

Summary

新建 CLI command: kploy validate-config — parses + validates a local kploy.yaml file and prints the hostnames it would render. Project owners can sanity-check their config locally before pushing.

Backed by bitcomplete/kployconfig. Tracks internal ticket KPL-19.

Usage

$ kploy validate-config --file kploy.yaml
OK

Sample hostnames:
ENV          HOSTNAME
prod         myapp.bitcomplete.dev
development  myapp-development.bitcomplete.dev

Example preview env (PR #1):
HOSTNAME                       SERVICE  PORT
myapp-pr-1.bitcomplete.dev     app      3000
myapp-pr-1-ws.bitcomplete.dev  socket   1999

The "Example preview env" section is only printed when preview envs are enabled in the file.

Errors propagate via cobra with a structured field path:

$ kploy validate-config --file bad.yaml
Error: project: must match ^[a-z][a-z0-9-]{2,12}$ (got "BadName")

--output json also supported, consistent with other commands.

What's in this PR

• cmd/validate_config.go — new command (~100 lines).
• cmd/root.go — register the command.
• README.md — new "Validating kploy.yaml" section.
• go.mod / go.sum — adds github.com/bitcomplete/kployconfig v0.1.1.

Test plan

• go build ./...
• go vet ./... / gofmt -l .
• Smoke: valid file with preview → text output renders cleanly
• Smoke: valid file without preview → no "Example preview env" section
• Smoke: --output json → consistent lowerCamelCase JSON
• Smoke: missing file → useful error message
• Smoke: invalid config → validation errors with field paths

[ivyrhodes]

Closing without merging — this work has re-landed in the kploy monorepo per the consolidation decision (bitcomplete/design-docs/0001-kploy-repo-structure.md).

The CLI source moved back into bitcomplete/kploy at cli/ (KPL-33 / https://github.com/bitcomplete/kploy/pull/108), and the validate-config command came along as part of that move. The validation library it depends on (kployconfig) folds in next under the re-scoped KPL-19.

bitcomplete/kploy-cli stays as the public release host so brew install bitcomplete/tap/kploy keeps working post-migration (per KPL-32). Source archival happens under KPL-34 once the monorepo migration is complete.