feat: bump the homebrew-tap formula automatically after each release

[bougyman]

Summary

• New homebrew-tap-bump job, parallel to the container job (both needs: [burrito-package]) - reads the just-published release's tag + SHA256SUMS, updates rubyists/homebrew-tap's Formula/linear-cli/linear-cli.rb url/sha256 for both platforms via ci/bump_homebrew_formula.rb, and opens a PR there via peter-evans/create-pull-request@v8 (no-op if nothing changed).
• Uses the RELEASE_PLEASE_TOKEN org secret to push/PR against a different repo than the one the workflow runs in - the default GITHUB_TOKEN is scoped only to this repo.
• The actual bump logic lives in ci/bump_homebrew_formula.rb rather than inline in the workflow YAML, so it's runnable/testable locally without a real CI run.
• Depends on feat: restructure the formula for per-package release-please versioning homebrew-tap#3 (submodule pointer bumped to that branch's tip - will need another bump if the SHA changes from a squash-merge once that PR lands).

Test plan

• Ran the bump script directly against a real copy of the formula and real release checksums (v1.2.0) - correct output, valid Ruby syntax
• Confirmed idempotent (re-running with the same version produces identical output)
• Re-verified after the tap repo's Formula/lc.rb -> Formula/linear-cli/linear-cli.rb move - still works correctly against the new path/structure
• Verified peter-evans/create-pull-request@v8's path: input (needed since this job checks out two different repos side by side) via its own docs
• actionlint clean on main.yaml
• mix format --check-formatted / mix usage_rules.sync --check clean in app/ (unaffected, but re-verified since they're now CI gates)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR automates updating the rubyists/homebrew-tap Homebrew formula after each rubyists/linear-cli release by adding a dedicated workflow job that downloads the release checksums, updates the formula’s per-platform url/sha256, and opens a PR in the tap repo.

Changes:

• Added ci/bump_homebrew_formula.rb to perform deterministic formula updates using the release tag and SHA256SUMS.
• Added a homebrew-tap-bump GitHub Actions job that checks out both repos side-by-side, downloads the just-created release’s SHA256SUMS, runs the bump script, and opens a PR via peter-evans/create-pull-request@v8.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
ci/bump_homebrew_formula.rb	Implements the formula bump logic (updates per-platform release asset URL + checksum in the formula file).
.github/workflows/main.yaml	Adds a post-release workflow job to run the bump script against rubyists/homebrew-tap and open a PR when changes exist.

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