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

docs/cli-reference.md

@@ -52,7 +52,8 @@ During fresh initialization, Fusion also installs the bundled `fusion` skill int
 
 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.
+a short next-steps tour. Each step can be individually skipped while staying in
+the onboarding flow.
 
 ```bash
 fn onboard