docs: move the reference out of the front door

The README was still twice the size of its JavaScript sibling at 1175
lines, and the difference was not extra capability. It was reference
material sitting on the front door: every reminder rule, every
configuration default, every CLI flag, and two registration examples,
all of which a reader passes on the way to deciding whether to install
anything.

Moved rather than deleted, because most of it had no other home.
docs/reminders.md is new and holds the keyed-alarm rules, the composed
name limit, the one-alarm-for-a-whole-queue drain, and reconciliation
for self-scheduling actors; docs/architecture.md now links there instead
of into a README heading. docs/operations.md gains the configuration
defaults table, the worker-count flags, the upgrade sequence, the
RuboCop exclusion, the Sorbet note, and the extension-component
contract. docs/architecture.md gains register_effect and
register_commit_action with their signatures.

Deleted only what was said twice. The table of contents duplicated the
headings GitHub already renders. A second example actor demonstrated
what the ticket sale demonstrates. Actor identity was three paragraphs
that fit in one, inside the section that defines an actor.

The README is now 528 lines and keeps what only it can say: the ticket
sale, the lock argument, who should not install this, the Durable
Objects mapping, and one worked example per capability with a link to
the guide behind it.

Author: cardmagic