FN-5805: add fn onboard command with sequential runOnboard flow

Add a new interactive onboarding CLI flow that guides first-time setup end to end.

- add new `fn onboard` command wiring in CLI entrypoint and usage help
- implement `runOnboard()` with sequential prompts for central DB, provider auth, init, test mode, and project maxConcurrent
- persist `cliOnboardingCompletedAt` marker in global settings with `--force` rerun support
- add onboarding command tests and global settings regression coverage
- document `fn onboard` usage and options in CLI reference
- add a minor changeset for published `@runfusion/fusion`

Files changed:
 .changeset/fn-5805-onboard-command.md              |   5 +
 docs/cli-reference.md                              |  20 ++
 packages/cli/src/bin.ts                            |  10 +
 packages/cli/src/commands/__tests__/onboard.test.ts| 193 ++++++++++++++++
 packages/cli/src/commands/onboard.ts               | 249 +++++++++++++++++++++
 packages/core/src/__tests__/global-settings.test.ts|  11 +
 packages/core/src/index.ts                         |   2 +-
 packages/core/src/settings-schema.ts               |   1 +
 packages/core/src/types.ts                         |   4 +
 9 files changed, 494 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-5805

Fusion-Task-Lineage: b1bcaf27-9bd7-4569-b685-225a97fa1200

docs/cli-reference.md

@@ -48,6 +48,26 @@ During fresh initialization, Fusion also installs the bundled `fusion` skill int
 
 ---
 
+## `fn onboard`
+
+Run the interactive CLI onboarding wizard. It walks through central DB setup,
+API-key provider setup, optional first-project init, core settings defaults, and
+a short next-steps tour.
+
+```bash
+fn onboard
+fn onboard --force
+```
+
+| Option | Description |
+|---|---|
+| `--force` | Re-run onboarding even when `cliOnboardingCompletedAt` is already set. |
+
+The command is safe to re-run and only updates the settings you confirm during
+prompts.
+
+---
+
 ## `fn update`
 
 Check for and install the latest `@runfusion/fusion` CLI release from npm.