Persona notices when a subject deserves a durable artifact

[LinseCed]

Dependencies

• Paired with sprintstart-backend#212, which sharpens the tool descriptions and supplies the
  repeat-subject signal. Neither half works alone.

Summary & Goal

Teach the buddy persona to notice when a subject was hard enough to deserve something durable, and
to reach for the board when it was.

The capability exists — the backend mounts board tools and executes them. What is missing is the
judgement about when, and that lives here: the backend describes the persona as being rebuilt
from the AI service's own skeleton on every turn with no system message
(sprintstart-backend · BuddyService.agentRequest).

Technical Specification

• The persona should treat a subject as worth a durable artifact when it took several turns to get
  through, when the hire has returned to it, or when the explanation will not survive the
  conversation. The backend supplies a repeat-subject signal per turn (sprintstart-backend#212) — use it as
  evidence, not as a trigger to obey blindly.
• Placing a card is not the answer to every hard question. A one-line clarification is not a
  diagram. Over-placing is the failure mode that makes a board unusable; say so in the persona rather
  than leaving it implied.
• DIAGRAM is the kind that fits an explanation, and it takes a subject — the question the
  picture should answer. The persona chooses that subject; the boxes come back derived from the
  project's corpus with a citation each, and an ungrounded box is dropped server-side. The persona
  must not assert what the diagram will contain.
• Once sprintstart-backend#210 lands, the buddy can also create notes and checklists. The persona should prefer the narrowest artifact that helps, and must say
  what it did — the hire is told, never surprised.
• Say it once. If a card is already on the board, point at it rather than placing again. The
  backend already answers ALREADY_THERE and DISMISSED_BY_HIRE in words, including
  "Do not add it again — if it matters, say it in the conversation instead." The persona should
  honour those rather than retry.

Acceptance Criteria

• The persona names the conditions under which a durable artifact is warranted
• It explicitly discourages placing a card for routine questions
• It uses the backend's repeat-subject signal as evidence rather than as an instruction
• It chooses a DIAGRAM subject without asserting the content
• It respects ALREADY_THERE and DISMISSED_BY_HIRE instead of retrying
• It tells the hire what it placed
• Tests with scripted LLM clients cover: hard subject → places, routine question → does not,
  already-placed → points instead of re-placing

Definition of Done

• uv run ruff check . clean
• uv run ruff format --check . clean
• uv run pyright src/ 0 errors
• uv run pytest green