[Feature]: Make the buddy the tutor for the reworked onboarding blueprint

[DavidLeuter]

Summary & Goal

The buddy and the onboarding path grew up separately. The path — now a PM-authored blueprint copied
per hire (#58) — is the plan. The buddy has its own world of arrival steps, starter work and the
board, and cannot see that plan at all: it has no tool that reads one. So a hire asking "what should
I do next?" gets an answer from somewhere other than the page they are looking at.

Bring the two together: the path stays the plan, and the buddy becomes the tutor along it. Its own
world stays too — it is what carries a hire where nobody has authored a blueprint — but it stops being
a second plan.

Depends on the three blueprint branches landing first (sprintstart-backend#58, sprintstart-ai#192,
sprintstart-frontend#217).

The line this rests on

• The blueprint is the PM's. The buddy never edits it.
• The hire's copy of the path is theirs. The buddy proposes; the hire confirms.
• Reading is free; every change goes through a confirm button.

What that means

• The buddy can see where the hire stands, what is open and what is locked, and talks about the phase
  they are in — not the whole plan, which is a page they already have.
• A knowledge question becomes a tutoring moment: it explains the material and is never told the
  correct answer, so it cannot hand one over.
• It can move the hire along on confirmation — finish a step, tick a line, send their answer, add a
  step to their own copy where a phase came back empty.
• Questions get routed: the path answers "what is next", the metrics and the ledger answer "how am I
  doing".
• The path becomes something both can point at — the buddy is reachable from every step and question,
  and names items the way the page numbers them.

Acceptance Criteria

• A hire with a path is told where they stand and one next thing; a hire without one meets a
  mentor that never mentions a path
• The buddy never states or hints at a correct answer, and says plainly that it does not have one
• A locked item is never agreed to; the reply says what it is waiting on
• Every change to the path arrives as a confirm button, and the blueprint is unreachable
• A step the buddy added says so, and the hire can edit or remove it
• A question about the path is never answered from the work pool

Definition of Done

• Checks pass in all three repos, and the three PRs are reviewed and merged into dev together —
  none of them ships alone