chore: release v0.11.0 (#168)

## Summary

Cuts release **v0.11.0**: bumps `package.json` from `0.10.0` to `0.11.0`
and promotes the `CHANGELOG`'s `[Unreleased]` section to `## [0.11.0]`.

`release.yml` verifies the pushed tag against `package.json`, so this
bump has to be on `main` *before* `v0.11.0` is tagged (#82).

**Minor bump**, not patch: this release adds four new command surfaces
(`files`, `emoji`, `team`, `usergroups`) and new flags on `messages` and
`conversations`. No breaking changes.

## Included since v0.10.0

**Added**
- `files info` / `read` / `download` — inspect, read and download Slack
files (#161, closes #157)
- `emoji list` / `emoji get` — read-only custom emoji (#149, closes
#140)
- `team info`, `usergroups list` / `read` — read-only, Enterprise Grid
aware (#148, part of #139)
- `usergroups create` / `update` / `add` / `remove` / `enable` /
`disable`, behind a `--yes` confirmation gate (#156, closes #139)
- `--json` and `--message-file` on `messages send` / `edit` / `draft`
(#152, closes #150)
- `--json` on `conversations list` (#153, closes #141)
- The project website at <https://slackcli.dev> (#165, closes #164)
- The Claude Code plugin with the `/slackcli` skill (#163, closes #162)

**Fixed**
- Slack API calls throttled through a shared rate limiter (#151, closes
#147)
- `findBrowser`'s PATH scan uses the platform's own path module (#155,
closes #154)

Seven of these merged without a `CHANGELOG` entry; this PR backfills
them.

## Changes in this PR

- `package.json`: `0.10.0` → `0.11.0`
- `CHANGELOG.md`: new `## [0.11.0] - 2026-09-05` section, `[Unreleased]`
left in place and empty

## Checks

- `bun run type-check` — clean
- `bun test` — 513 pass, 0 fail
- pre-commit hooks — passed, commit signed

## After merge

Push the annotated tag `v0.11.0` at the merged bump commit, which
publishes the binaries and updates the Homebrew tap.

---

This is a release PR: it proposes no new behaviour, only the version
bump and the changelog promotion for changes already reviewed and
merged. Labelled `no-issue-needed` rather than linked to an issue.