feat(onboarding): add interactive CLI UX with banner, prompts, progress

New module src/onboarding.rs providing three public entry points:
- run_interactive: full first-run experience with banner, vault scan,
  dialoguer prompts (name/role/purpose), indexing, L1 extraction
- run_detect_json: non-destructive vault inspection returning JSON
- run_apply_json: non-interactive setup for agent/CI use

Supports flags for partial flows (identity-only, reindex-only, quiet)
and pre-filled values to skip interactive prompts.

Author: devwhodevs

src/lib.rs

@@ -14,6 +14,7 @@ pub mod llm;
 pub mod markdown;
 pub mod migrate;
 pub mod obsidian;
+pub mod onboarding;
 pub mod openapi;
 pub mod placement;
 pub mod profile;