Document the journey workflow: analyse-journey command + auto-publish behavior

[bmichele]

Problem

The CLI documentation has a gap around the journey workflow:

1. analyse-journey is undocumented. docs/reference/cli.md bills itself as a "Complete reference for every skene command and flag," but the analyse-journey command has no section there, nor in docs/guides/, nor in any examples — even though it's a real command and the TUI drives it. This predates the auto-publish work.

2. Auto-publish behavior needs proper documentation. PR Auto-publish journey.yaml to Skene Cloud on first analysis #85 adds a new user-observable behavior: running journey analysis from the TUI while linked to a workspace auto-publishes journey.yaml to Skene Cloud when none exists upstream. That PR added only a minimal stopgap note in docs/guides/push.md ("Automatic first publish") and docs/getting-started/quickstart.md. There's no dedicated home for the journey flow (TUI → visualization → skene.ai canvas).

Suggested scope

• Add an analyse-journey section to docs/reference/cli.md (arguments, options, behavior notes), matching the style of the other command sections. Note: the --auto-publish flag is intentionally hidden/internal and should not be listed as a user-facing flag — document the behavior, not the flag.
• Add a journey guide (e.g. docs/guides/journey.md) covering: generating journey.yaml, the TUI visualization, the link to the skene.ai Customer Journey canvas, and the auto-publish-on-first-run behavior (with its gating: skene provider + linked workspace + no journey upstream).
• Link the new guide from docs/index.md and the quickstart, and replace/expand the stopgap notes added in Auto-publish journey.yaml to Skene Cloud on first analysis #85.

Context

• Stopgap notes landed in Auto-publish journey.yaml to Skene Cloud on first analysis #85 (push.md + quickstart.md).
• Auto-publish gating logic: src/skene/cli/commands/analyse_journey.py (_maybe_auto_publish), presence check in src/skene/growth_loops/upstream.py (journey_exists_upstream), TUI opt-in in tui/internal/services/growth/engine.go (journeyArgs).