legal: repository has no LICENSE — blocks adoption; needs a copyright-holder decision

The repository has **no `LICENSE` file** (GitHub reports `license: null`). For a public SDK this is "all rights reserved" by default: no platform team can legally adopt it, `pkg.go.dev` shows no license, and the new README landing page (#68) deliberately omits a license line rather than invent one. This blocks the project's entire purpose until resolved.

## Why this needs a human

License selection is a decision for the copyright holder (Coder), not the maintainer. Labeled `ready-for-human`.

## Recommendation

Coder's org convention is consistent: **libraries are permissive** (`coder/websocket` ISC, `coder/quartz` MIT-0, `coder/serpent`/`coder/pretty`/`coder/guts` CC0-1.0) and **products are AGPL-3.0** (`coder/coder`, `coder/mux`). Chat SDK Go is an embeddable library, so it belongs in the permissive group. Suggested: **MIT** (most familiar to Go platform teams and to dependency-license scanners) or **Apache-2.0** (adds an explicit patent grant). Either is fine; AGPL would defeat the adoption goal.

## Once decided

- Add `LICENSE` at repo root (GitHub's license picker generates the standard text).
- Add a one-line License section to README.md.
- Verify `pkg.go.dev` picks it up after the next tag.

I will execute the mechanical part via the standard PR gate as soon as a license is chosen here.

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