README for the release, and ADR-0003

Parent

Bring the README up to date for the first release, and record the release decision as an ADR.

Status: the designed behaviour is implemented; known gaps are tracked in open issues. Remove the stale "except marking a draft ready" caveat from the steps of a Run.
Install: lead with the curl installer from GitHub Releases, installing to ~/.local/bin, with no Rust needed. Mention cargo install thirdshift only as an alternative for Rust users.
Prerequisites: claude, gh and git only. The Rust toolchain moves to a new "Building from source" section.
Supported platforms: Rocky Linux 9.8+ (x86_64), Ubuntu 24.04.3+ (x86_64), WSL2 latest with Ubuntu 24.04+ (repositories on the Linux filesystem, not /mnt/c; WSL1 not supported), and macOS Tahoe 26.5.2+ (Apple Silicon).
Releasing: bump the version in a pull request, merge it, then push a v<version> tag.
Usage: covers thirdshift <Issue URL>, update, version and help (bare words only).
ADR-0003: thirdshift ships as static musl binaries through dist GitHub Releases with a curl installer and built-in updater, mirrored to crates.io, instead of crates.io alone. Record why: no Rust needed for the install command, glibc on Rocky, and the permanence of the crate name.

CONTEXT.md is unchanged.

Status, Install, Prerequisites, Usage and Updating reflect the shipped behaviour.
The stale draft-ready caveat is gone.
The Supported platforms section lists all four platforms, with the WSL2 caveats.
The Releasing section gives the three steps.
ADR-0003 exists in the repository's ADR format and records the trade-off.