chore: release v0.10.0 (#142)

## Summary

Cut release **v0.10.0** — bump `package.json` from `0.9.1` to `0.10.0`
and promote the `[Unreleased]` CHANGELOG section under a `## [0.10.0] -
2026-08-29` heading.

The bump has to be on `main` *before* the tag is pushed: `release.yml`'s
`verify-version` step fails when the tag disagrees with `package.json`
(#82).

## Why a minor bump

`0.x` SemVer as this repo uses it: a new capability is a minor, fixes
only are a patch. #124 adds a new flag, so `0.9.1` → `0.10.0`.

## Included since v0.9.1

**Added**
- Native Block Kit messages — `messages send --blocks <json|@file>`
passes a JSON array of structured blocks to `chat.postMessage`,
including Slack's native `table` blocks with rich-text cells and
`markdown` blocks. Works with standard and browser-session auth (#121,
#124)

Everything else merged since `v0.9.1` is documentation only (#126, #128,
#131, #133, #134, #135, #136) — no shipped behaviour, so nothing else in
the changelog.

## Changes in this PR

- `package.json`: `0.9.1` → `0.10.0`
- `CHANGELOG.md`: new `[0.10.0]` heading; `[Unreleased]` left in place
and empty

## Checks

- `bun run type-check` — clean
- `bun test` — 422 passed, 0 failed
- pre-commit hooks — passed

## After merge

Push the annotated tag `v0.10.0` at the merged bump commit, which
triggers `release.yml` to build the binaries and update the Homebrew
tap.

This PR is labelled `no-issue-needed`: a release proposes nothing new —
everything in it was already authorized when it merged.