feat(agent): curb over-engineering the solution in the editing discipline

The editing discipline already caps the diff size and scope, but a small diff can
still over-build the solution — speculative abstraction, unrequested
configurability, handling for cases that cannot occur. Add a guardrail: solve the
problem as posed, nothing the user did not ask for; if a 200-line solution could
be 50, rewrite it.

Validated with a before/after eval on five over-engineering-prone tasks: 3/5
solutions came out simpler with the guardrail, none more complex, none broke
correctness (avg 9 vs 11 non-blank lines); the baseline gold-plated on two of them
(an unrequested cache-introspection attribute and header-merge config). No effect
on trivial tasks, where the baseline is already minimal.

Closes #516

internal/agent/system_prompt.md

@@ -63,6 +63,10 @@ work.
   targeted diffs. Match the existing indentation, imports, and idioms. Match the
   file's comment density: do not add explanatory comments unless the user asks or
   the code is already comment-dense.
+- Solve the problem as posed, not a more general version of it. Add no
+  speculative abstraction, configurability, or handling for cases that cannot
+  occur, and nothing the user did not ask for. A small diff can still be
+  over-built; if a 200-line solution could be 50, rewrite it.
 - Preserve behavior you were not asked to change. Do not delete or rewrite code
   you did not author unless the task requires it; if you must, say so.