Prepare the 0.2.0 release: changelog, README, package doc

[ExaltedTrou6]

The changelog section becomes 0.2.0 — it describes what is already on main,
and calling it Unreleased would be wrong the moment the tag is placed.

README and doc.go still described v0.1.0 only. Neither mentioned Params,
Kind, WithDescending, WithCallOptions or WithTimeout — so everything the
tutorial run turned up was invisible to anyone arriving at the repository or at
pkg.go.dev, which is the first screen most readers ever see.

Added to both, each in the place a reader is already looking:

• Params next to the call that takes it, with the alias-ness spelled out —
  that is what lets it mix with a plain map[string]any.
• Kind next to IsEmpty, because the pair splits one question in two: is
  anything there, and what is it.
• WithDescending in the walk section, with the warning NOT to reverse a
  Scan by hand: flipping only the order gives order DESC + filter >id,
  which asks for rows above the newest one already seen, so the walk stops after
  page one and reports a complete export.
• WithCallOptions / WithTimeout right after it, with the gap they close —
  a walk takes one ctx for the whole loop, so otherwise the only choices are a
  deadline on the entire export or none at all on a page that stalls.
• For the batch: commands run in submission order, and per-command
  next/total live in res.Next[id] rather than inside the result, where it
  is the KEY'S PRESENCE that means "there is more".

README also gains what a public repository is expected to have and did not: a
go get line, the required Go version, CI and pkg.go.dev badges, and a pointer
to llms.txt for people writing integrations with an AI agent.

Merges cleanly with #16 in either order; CI checked on the combined result.

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