feat: add Nix flake packaging and prepare README for open-source

[kylecarbs]

Packages boo as a Nix flake and restructures the README for open-sourcing, following the structure of the coder/coder and coder/code-server READMEs.

Changes

• flake.nix / flake.lock: installable package (nix run github:coder/boo, nix profile add github:coder/boo), a dev shell with Zig 0.15.2, and nixfmt-tree as the formatter. The version is parsed from build.zig.zon so there is one source of truth.
• README: centered header with tagline, quick links, and badges (CI, release, license, Discord); split Install into install-script and Nix sections; documented nix develop / nix build; added Support and Contributing sections.
• CI: new nix job running nix flake check, nix build, and a nix run . -- version smoke test.
• .gitignore: ignore Nix result symlinks.

Implementation notes

• Nix builds are sandboxed without network access, so all Zig dependencies pinned in build.zig.zon (libghostty and its transitive deps) are pre-fetched with zig build --fetch=all into a fixed-output derivation, mirroring what CI does. Instructions for updating outputHash when deps change are in a comment in flake.nix.
• The dependency cache is copied into ZIG_GLOBAL_CACHE_DIR, not symlinked: ghostty's uucode table generator runs helper executables with a working directory inside the package cache and locates outputs via relative ../ paths, which resolve physically into /nix/store through a symlink and fail with FileNotFound.
• The package builds with zig.hook defaults (--release=safe -Dcpu=baseline), matching the ReleaseSafe mode of published release binaries, and runs zig build test (unit tests) in the check phase. PTY integration tests remain in the existing CI jobs.
• nixpkgs-unstable is pinned because it ships Zig 0.15.2 exactly, satisfying minimum_zig_version.

Verification

On x86_64-linux:

• nix flake check passes.
• nix build succeeds; check phase runs unit tests.
• ./result/bin/boo version prints boo 0.5.12.
• Full automation loop against the Nix-built binary: new -d, send --text --enter, wait --idle, peek (output observed), kill.
• nix run . -- version works.
• nix fmt is a no-op (flake already formatted).

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This PR was generated by Coder Agents on behalf of @kylecarbs.