feat(release): install shell completions on brew install (DX-5783)

[johnpmitsch]

Summary

brew install quicknode/tap/qn installed the binary but no shell completions, so brew users got no tab-completion out of the box. qn completions <shell> already generates working bash/zsh/fish completions (pure local generator — no key, no network), but the formula never invoked it.

The formula is generated by cargo-dist and copied verbatim into quicknode/homebrew-tap each release, so it can't be hand-edited durably; cargo-dist's template also dumps any bundled files into pkgshare, where shells don't read them. So this patches the generated formula in the release-update-homebrew-tap recipe (the one place that owns formula → tap), inserting generate_completions_from_executable(bin/"qn", "completions") after the binary install. At brew install time that runs qn completions bash|zsh|fish and drops each in the right dir.

The patch is idempotent (skips if already present) and fails loud if cargo-dist's install_binary_aliases! anchor disappears, so a future template change can't silently drop completions from a release.

This is Justfile-only — no src/ change.

Test plan

Verified the patch logic against the live Formula/qn.rb without doing a release:

• Patch inserts the helper inside def install right after install_binary_aliases!; ruby -c on the result → Syntax OK.
• Idempotent: running the patch twice leaves exactly one helper line.
• Fail-loud: with the anchor removed, the recipe exits non-zero with a descriptive message and leaves the file (and no .tmp) untouched.
• Anchor regex targets the call site, not the def install_binary_aliases! definition.
• just parses the recipe; cargo fmt --check clean.

Post-merge, the next release's formula will carry the completion line; a fresh brew install/upgrade then yields qn <TAB> completion in zsh/bash/fish.

Closes DX-5783