docs: add a runnable custom conversion rule

[noeltock]

Problem

Developers can supply custom HTML conversion rules, but there was no runnable example showing the hook or its limits. Closes #109.

Solution

Add a small paragraph rule, a guide with the actual CLI output, and a test that imports the shipped example.

Behaviour	Read first	Proof
Marked paragraphs become native paragraphs with a notice class	examples/custom-rule.mjs	CLI example and focused test
Unmarked input keeps normal conversion	dev/test/custom-rule-example.test.ts	Control paragraph assertion
Rules retain the normal style and validation steps	docs/extending.md	Source review and zero-invalid output

Diff

+89 −0 · 3 files · no runtime or API changes

+ examples/custom-rule.mjs
+   p[data-notice] -> core/paragraph with className: notice
+ docs/extending.md
+   runnable command, output, precedence and limitations
+ dev/test/custom-rule-example.test.ts
+   imports the example; checks marked and unmarked native output

Testing & verification

Reviewed revision: 861201e0acec0b573845357e7dc728a884239ab9 · Environment: macOS, Node 22.23.1; existing matching dependencies reused.

• npm run build — passed.
• npx vitest run dev/test/custom-rule-example.test.ts dev/test/convert.test.ts — passed, 2 files / 6 tests.
• npx --no-install block-runner convert '<p data-notice>Service update</p>' --config examples/custom-rule.mjs — passed; emitted the documented native paragraph with the notice class.
• npm run typecheck and npm run check:private — passed.
• npm run pack:check — passed; includes examples/custom-rule.mjs.
• git diff --cached --check — passed before commit. Reviewed the new guide separately for private references because the packed-file scan excludes docs/.

A fresh installed consumer also ran npx --no-install block-runner convert '<p data-notice>Service update</p>' --config node_modules/block-runner/examples/custom-rule.mjs --json — passed with one valid native paragraph, zero invalid blocks and no warnings. Its resolved dependency tree emitted class="notice notice"; the locked checkout emits class="notice". The reported block attribute is className: "notice" in both. The guide shows the observed checkout output; this PR does not change serialization.

Not verified: real-WordPress rendering and third-party block registration. The example establishes conversion and headless validity only; it does not generate CSS. GitHub CI: all three Node lanes and the package check passed. The first WordPress run failed its native style-adapter editor check: the retained receipt records a 1278px viewport where 1280px was required. Attempt 1 evidence is retained. The same runtime code passed on #113. gh run rerun 34981527836 --failed --repo humanmade/block-runner — passed in attempt 2. All checks passed on the unchanged revision; no test threshold or runtime source was changed.

Risk / rollout

The hook forwards this example's simple paragraph content; the guide does not promise sanitisation or arbitrary rich-text support. No dependencies, website files or default rules change.

Detection: the test imports the actual example and checks both inputs. Rollback: revert this commit.

Authored by: GPT-6 via Codex.