docs: plain up prose in README, guides, CONTEXT and AGENTS

[lesnik512]

Prose-only edit of the README, the docs pages, CONTEXT.md and AGENTS.md to remove AI-writing tells. No facts, option names, defaults, numbers or link targets change, and no code blocks were touched.

Most of the change replaces em dashes with a colon, semicolon, comma or a new sentence, depending on how the clauses relate. Other edits:

• README: "handy when poking around in redis-cli" now says plainly that those are the keys to look up there.
• CONTEXT.md: dropped "and the difference matters" from the Topic entry, and split the Schedule entry so the reason for the publish() name is one sentence and the domain verb is another.
• docs/index.md and docs/usage/subscriber.md each had one dash and got the same treatment, to stay consistent with the README.

Left alone on purpose:

• Dashes inside code comments in basic.md and publisher.md (code).
• The — in the message row of the publisher options table, which marks an empty default.
• The ~3–5× range in the lease_ttl row of how-it-works.md.
• The emoji link headings at the bottom of the README, which follow the org-wide template.
• ADRs: no tells worth an edit.
• Contrasts in CONTEXT.md that carry information ("not the message it carries", "rather than exactly-once", "A Claim is not a lock").

mkdocs build --strict passes.