docs: thread code-review through the full build-chain flow

The build chain now ends at code-review. Update every flow doc that draws
the chain so it reads end-to-end:

    grill-with-docs → to-prd → to-issues → implement → code-review

- grill-with-docs / to-prd / to-issues: extend the diagram; point the
  downstream neighbour at implement (which drives tdd internally).
- tdd: reframe from "final step" to the engine inside implement's step.
- implement: extend the diagram to include the review pass.
- ask-matt: narrate the flow ending in review (docs) and note that
  /implement closes out with /code-review (SKILL.md).
- writing-docs guide: refresh the example chain.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/engineering/grill-with-docs.md

@@ -44,7 +44,7 @@ What makes this variant its own skill is where the answers go. As the grill runs
 `grill-with-docs` is the opening step of the main build chain:
 
 ```txt
-grill-with-docs → to-prd → to-issues → implement
+grill-with-docs → to-prd → to-issues → implement → code-review
 ```
 
 It comes first, before anything is written down as a spec: it produces the shared understanding and settled vocabulary that [to-prd](https://aihero.dev/skills-to-prd) then synthesises into a PRD without re-interviewing you. Its close neighbours are [grilling](https://aihero.dev/skills-grilling), the same interview without the docs, and [domain-modeling](https://aihero.dev/skills-domain-modeling), the glossary-and-ADR discipline it drives. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.