practice: target-validation pins + the id convention in the base prompt (round 9 slice 3)

Four pins on the mastery-trace contract: a name target normalizes to
the canonical id (case-insensitive, whole-graph set), an id passes
through untouched, an unknown target rejects one-line AND does not arm
the gate (honest activation — the next valid pose goes through), and
the legacy node-source-less wiring stays unvalidated. The base prompt
now states the convention: target takes the proposition ids from the
session context — ids are how problems are traced.

src/agent/prompts/practice/00-base.md

@@ -23,7 +23,7 @@ He has to be able to trust you completely; one confidently-delivered hallucinati
 
 Two tools, and everything else is conversation. The mechanics only — the method (when to pose, what to pose, how to respond) is the doctrine's.
 
-**`problem`** — poses one problem. The statement field is what the learner sees (markdown and LaTeX render); the modelSolution is your private commitment, hidden from the learner until you conclude. The call ends the turn. The learner then works in a thinking field; each submission arrives back as a *Your thinking* message. Whether it claims an answer is IN THE TEXT, not declared anywhere — read it:
+**`problem`** — poses one problem. The statement field is what the learner sees (markdown and LaTeX render); the modelSolution is your private commitment, hidden from the learner until you conclude. The target field takes the proposition ids from the session context — ids are how every problem is traced to the propositions it exercises. The call ends the turn. The learner then works in a thinking field; each submission arrives back as a *Your thinking* message. Whether it claims an answer is IN THE TEXT, not declared anywhere — read it:
 
 - Working trace (mid-work, stuck, or asking for a nudge): the hint ladder applies, one rung per request. A trace may span several submissions.
 - A claimed answer: extract the moves, deliver the verdict. A claim is not a progress signal.