Rewrite Implementer prompt around a scoped execution contract

[jrpbuilds]

Parent

Part of #46.

Relationships

Depends on #55 and #56.

Goal

Rebuild the Implementer prompt around the minimum semantic contract needed to execute approved work safely and accurately, now that assignment legality and current-job context are enforced/supplied outside the prompt.

Keep

The new prompt should establish that the Implementer:

• implements the approved OpenSpec change/task scope;
• inspects source and tests directly rather than relying on summaries;
• follows approved requirements/design and repository conventions;
• treats assignedTaskIds, when present, as the entire scoped assignment;
• does not silently redesign approved requirements/design when a conflict is discovered;
• verifies changed behaviour appropriately;
• marks only genuinely completed OpenSpec tasks complete;
• reports blockers/conflicts rather than fabricating completion;
• returns the required handoff/result contract.

Remove/reduce

Once #55/#56 exist, remove detailed prompt instructions for:

• sibling assignment overlap and disjointness;
• concurrency/capacity accounting;
• lane scheduling/refill policy;
• progress-reporting checkpoints;
• broad background-dispatch mechanics;
• resumed-session assignment validation that the runtime can perform against fresh state;
• detailed scheduler rationale for lane-focused tests;
• current task IDs/apply context/findings that belong in the dispatch payload.

Keep genuinely useful implementation/testing judgement guidance, but avoid explaining the Coordinator's orchestration machinery to the Implementer.

Remediation

Ensure the same compact prompt works for ordinary implementation and review remediation when the dispatch payload clearly supplies the relevant finding/correction target. Add only the semantic remediation rules that the Implementer genuinely needs.

Acceptance criteria

• Implementer prompt is rebuilt from a minimal scoped-execution contract.
• Scoped assignedTaskIds behaviour remains explicit and reliable.
• Illegal/stale/overlapping assignments are handled by runtime validation rather than prompt tutorials.
• Approved design/spec conflicts are surfaced rather than silently redesigned.
• Task completion remains tied to real implementation and verification.
• Remediation works through dynamic finding context without a second large workflow manual.
• Prompt contains no Todo/progress/capacity scheduler protocol.
• Serial, scoped-parallel, resumed and remediation scenarios retain correct behaviour.
• Final prompt size is substantially below v1.6.

Out of scope

• Automatic lane formation.
• Coordinator/reviewer rewrites.
• Final prompt-budget thresholds.