FN-5806: make onboarding steps individually skippable

Allow each fn onboard stage to be skipped without exiting the overall onboarding flow.

- add a reusable skippable-step helper that prompts before each onboarding section
- gate central DB setup, provider setup, project init, core settings, and next-steps output behind per-step skip prompts
- preserve onboarding completion persistence while keeping cancellation semantics intact
- expand onboarding command tests for full skip, selective skip, and cancellation coverage
- document per-step skip behavior in the CLI reference
- add a patch changeset for @runfusion/fusion

Files changed:
 .changeset/fn-5806-onboard-step-skip.md            |   5 +
 docs/cli-reference.md                              |   3 +-
 packages/cli/src/commands/__tests__/onboard.test.ts     |  72 ++++++++++++-
 packages/cli/src/commands/onboard.ts               | 112 ++++++++++++---------
 4 files changed, 141 insertions(+), 51 deletions(-)

Fusion-Task-Id: FN-5806

Fusion-Task-Lineage: d122d67a-2a13-4b2f-a1aa-459dc548d370

.changeset/fn-5806-onboard-step-skip.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+`fn onboard` now allows each onboarding step to be skipped individually without aborting the overall wizard. Skipping steps still marks onboarding as completed, while interactive cancellation behavior remains unchanged.