docs: name katoptra/dispatch as the scheduler

[jshvn]

What

The README described the thing that starts every run as "a scheduler" without naming it, and the 2026-09-08 design spec named jshvn/dispatch, which stopped scheduling katoptra mirrors on 2026-09-21. Now:

• Both README diagrams and the two sentences about the scheduler name katoptra/dispatch and link to it.
• The spec's clock row says katoptra/dispatch, with a one-line dated note under the table that it was jshvn/dispatch when the spec was written and that the contract in the row is unchanged.

Docs only.

Test plan

• grep -n "jshvn/dispatch" over the repo: only the dated note in the spec
• Mermaid blocks still parse (labels changed, edges did not)