FN-5812: add onboard autolaunch backcompat guard coverage

Protect onboarding autolaunch behavior so legacy projects and agent-run commands are not blocked.

- add end-to-end CLI test coverage for onboarding autolaunch backcompat guard behavior
- document the onboarding autolaunch compatibility behavior in the CLI reference
- add a patch changeset for @runfusion/fusion release tracking

Files changed:
 .changeset/fn-5812-onboard-backcompat-guard.md     |   5 +
 docs/cli-reference.md                              |   6 +
 packages/cli/src/commands/__tests__/onboard-autolaunch-backcompat-e2e.test.ts      | 161 +++++++++++++++++++++
 3 files changed, 172 insertions(+)

Fusion-Task-Id: FN-5812

Fusion-Task-Lineage: d735ef0e-1abc-4c22-bd11-07c2de69ace1

docs/cli-reference.md

@@ -67,6 +67,12 @@ fn onboard --force
 The command is safe to re-run and only updates the settings you confirm during
 prompts.
 
+Backward compatibility guarantee: onboarding auto-launch never interrupts
+existing setups and never blocks headless usage. It is skipped when both the
+central DB and local project DB already exist, and it never prompts in non-TTY,
+`serve`, `daemon`, `--skip-onboarding`, `FUSION_SKIP_ONBOARDING`, piped, or
+agent-run invocations.
+
 ---
 
 ## `fn update`