feat: apt repo and install script installation methods

[emberian]

Summary

• Debian/Ubuntu: Installs Elide via the official apt repository (GPG key + sources list + apt-get install), avoiding tarball downloads on runners that support it
• macOS / non-Debian Linux: Installs via the elide.sh install script, downloaded to a temp file and executed with bash
• Windows / custom_url: Unchanged -- continues using the existing tarball/zip download path
• CI test-action job now runs on an ubuntu-latest, macos-latest, windows-latest matrix

New files

• src/platform.ts -- detects Debian-like systems via /etc/debian_version
• src/install-apt.ts -- apt repo installer with set -o pipefail for safe GPG key download
• src/install-script.ts -- install script path with ~/.elide/bin PATH fallback
• __tests__/platform.test.ts, __tests__/install-apt.test.ts, __tests__/install-script.test.ts -- 8 new unit tests

Test plan

• Verify pnpm test passes for new test suites (platform, install-apt, install-script)
• Verify CI matrix runs on all three OS targets
• Verify custom_url input still routes to tarball download (not apt/script)
• Verify apt installer maps aarch64 -> arm64 for Debian arch naming

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 81.63265% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.86%. Comparing base (d91d585) to head (7b7514a).

Files with missing lines	Patch %	Lines
src/main.ts	46.66%	4 Missing and 4 partials ⚠️
src/releases.ts	60.00%	8 Missing ⚠️
src/command.ts	0.00%	1 Missing ⚠️
src/install-script.ts	96.29%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #232       +/-   ##
===========================================
- Coverage   91.06%   76.86%   -14.20%     
===========================================
  Files           7       10        +3     
  Lines         179      268       +89     
  Branches       21       38       +17     
===========================================
+ Hits          163      206       +43     
- Misses         11       54       +43     
- Partials        5        8        +3     

Files with missing lines	Coverage Δ
src/install-apt.ts	100.00% <100.00%> (ø)
src/platform.ts	100.00% <100.00%> (ø)
src/command.ts	55.55% <0.00%> (-44.45%)	⬇️
src/install-script.ts	96.29% <96.29%> (ø)
src/main.ts	78.46% <46.66%> (-17.77%)	⬇️
src/releases.ts	50.63% <60.00%> (-27.50%)	⬇️

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