Stop smuggling per-call agent arguments through workflow state

[czpython]

Three fields on build's workflows were arguments to the next agent call dressed up as run state — set by mutating self so get_prompt_context / the workspace hooks could read them back:

• _answered / _note carried the operator's gate reply into exactly one plan pass, then reset.
• Profile._repo smuggled what input.repo_id already determines.

Agent calls already have the right channel: call kwargs merge into the template namespace at top level (the repo_profiler(repo=...) precedent). So:

• generate_plan passes answered_questions= / operator_note= as call kwargs; the instance fields, their context plumbing, and the BuildPromptContext fields are deleted. generate_plan.md reads the kwargs top-level (build.answered_questions → answered_questions, build.operator_note → operator_note).
• Profile resolves ProjectRepo.get(self.input.repo_id).full_name in each hook instead of stashing it on the instance.

Behavior identical; prompts render the same context. One note for repo-level prompt overrides of generate_plan.md: the two names lost their build. prefix, and StrictUndefined fails loudly on a stale override rather than rendering empty prose.

Lands before the BuildJournal (ENG-726) so the journal doesn't inherit the smuggle fields.

ENG-725