Lift superpowers:code-reviewer agent into the requesting-code-review skill

The plugin had a single named agent (`agents/code-reviewer.md`) used by
two skills, while every other reviewer/implementer subagent in the repo
is dispatched as `general-purpose` with the prompt template living
alongside its skill. That asymmetry had no upside and several costs:

- Two sources of truth for the code review checklist (the agent file
  and `requesting-code-review/code-reviewer.md`), both drifting
  independently.
- `Codex` users could not use the named agent directly; the codex-tools
  reference doc had a workaround section explaining how to flatten the
  named agent into a `worker` dispatch.
- No third-party reliance on `superpowers:code-reviewer` inside this
  repo.

Changes:
- Merge `agents/code-reviewer.md` (persona + checklist) and
  `skills/requesting-code-review/code-reviewer.md` (placeholder
  template) into a single self-contained Task-dispatch template,
  matching the shape of `implementer-prompt.md`,
  `spec-reviewer-prompt.md`, etc.
- Update `skills/requesting-code-review/SKILL.md` and
  `skills/subagent-driven-development/code-quality-reviewer-prompt.md`
  to dispatch `Task (general-purpose)` instead of the named agent.
- Drop the now-obsolete "Named agent dispatch" workaround sections from
  `codex-tools.md` and `copilot-tools.md` — superpowers no longer ships
  any named agents, so those instructions documented nothing.
- Delete `agents/code-reviewer.md` and the empty `agents/` directory.

Tier 3 coverage for the change: a new behavioral test
`tests/claude-code/test-requesting-code-review.sh` plants real bugs
(SQL injection, plaintext password handling, credential logging) into
a tiny project, runs the actual `requesting-code-review` skill against
the working tree, and asserts the dispatched reviewer flags every
planted issue at Critical/Important severity and refuses to approve
the diff.

Verified end-to-end on this branch:
- The new test passes (5/5 assertions; reviewer caught all planted
  bugs and several others).
- The existing SDD integration test still passes (7/7 subagents
  dispatched, all as `general-purpose`; spec compliance still
  rejects extra features; produced code is correct).
- Session JSONLs confirm zero remaining `superpowers:code-reviewer`
  dispatches anywhere in the SDD pipeline.

skills/subagent-driven-development/code-quality-reviewer-prompt.md

@@ -7,14 +7,13 @@ Use this template when dispatching a code quality reviewer subagent.
 **Only dispatch after spec compliance review passes.**
 
 ```
-Task tool (superpowers:code-reviewer):
+Task tool (general-purpose):
   Use template at requesting-code-review/code-reviewer.md
 
-  WHAT_WAS_IMPLEMENTED: [from implementer's report]
+  DESCRIPTION: [task summary, from implementer's report]
   PLAN_OR_REQUIREMENTS: Task N from [plan-file]
   BASE_SHA: [commit before task]
   HEAD_SHA: [current commit]
-  DESCRIPTION: [task summary]
 ```
 
 **In addition to standard code quality concerns, the reviewer should check:**