docs: mention Homebrew install option

[rursache]

Summary

• OpenSpec is published on Homebrew (https://formulae.brew.sh/formula/openspec) but the install docs never mention it
• Add brew install openspec under npm in README.md (Quick Start + Updating) and docs/installation.md

Test plan

• Render README.md and docs/installation.md on the fork to confirm formatting

Summary by CodeRabbit

• Documentation
  • Added Homebrew installation and upgrade options for macOS and Linux users, enabling package manager-based installation and updates alongside existing npm global commands.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 9a39ad56-ae6e-4589-ba8c-2d0c147451d9

📥 Commits

Reviewing files that changed from the base of the PR and between 6bce907 and 322974b.

📒 Files selected for processing (2)

• README.md
• docs/installation.md

🚧 Files skipped from review as they are similar to previous changes (1)

• docs/installation.md

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📝 Walkthrough

Walkthrough

README and docs/installation.md add Homebrew installation guidance, while README_OLD.md is removed.

Changes

Installation documentation

Layer / File(s)	Summary
Homebrew installation and upgrade guidance README.md, docs/installation.md	Adds brew install openspec and brew upgrade openspec alongside existing npm commands and documents Homebrew under package managers.
Legacy documentation removal README_OLD.md	Removes the former legacy documentation content.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Possibly related PRs

• Fission-AI/OpenSpec#574: Updates related OpenSpec installation documentation.

Suggested reviewers: tabishb

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the docs change adding Homebrew install instructions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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