feat: GitHub integration with OAuth Device Flow

[corvid-agent]

Summary

• Add full GitHub integration: pull specs from a repo, push changes to branches, and create PRs — all via the GitHub REST API
• Add GitHub OAuth Device Flow (RFC 8628) as the primary auth method with a "Sign in with GitHub" button, keeping PAT as a fallback
• Add mobile-responsive layout, CI spec validation workflow, and spec-check CLI tooling

Changes

GitHub Integration (99e24bc)

• GitHubService — REST API client with PAT auth for pulling .spec.md files, pushing changes to new branches, and creating PRs
• GitHubConnectComponent — sidebar panel for repo connection (owner/repo/branch/path config, pull/disconnect controls)
• Editor page — "Create PR" button for GitHub-sourced specs
• Spec model — githubSha field for tracking remote blob state
• Specs for github-service and github-connect, updates to 5 existing specs

OAuth Device Flow (f94c0b1)

• GitHubOAuthService — full device flow lifecycle: device code request, token polling with slow_down/expired_token handling, user profile fetch, localStorage persistence
• Environment config — GITHUB_CLIENT_ID for the OAuth App
• GitHubConnectComponent overhaul — four UI states: sign-in, device code display, authenticated + repo config, connected
• GitHubService — falls back to OAuth token when no PAT configured, connectWithOAuth() method

Mobile & Tooling (293aa04)

• Mobile-responsive layout with sidebar overlay and section nav toggle
• spec-check CI workflow (.github/workflows/ci.yml) validates .spec.md on push/PR
• spec-check CLI tool and npm script
• CLAUDE.md agent instructions and .speckit/ constitution
• Fix validator false-positive for sections with subsections

Test plan

• Verify npm run spec-check passes all 15 specs
• Verify ng build succeeds with no errors
• Test GitHub PAT flow: enter token, connect to repo, pull specs, disconnect
• Test OAuth Device Flow: click "Sign in with GitHub", verify user code appears, complete authorization, confirm avatar/username shown
• Test "Create PR" from editor page with a GitHub-sourced spec
• Test mobile layout: sidebar overlay, section nav toggle

🤖 Generated with Claude Code

[corvid-agent]

@corvid-agent rebase and fix the merge conflicts

[corvid-agent]

Rebased feat/github-integration onto main.

Two of the three commits were already upstream (from PR #1 merge) and were dropped automatically — no conflicts to resolve. The branch now contains a single commit:

• 0e08a5a feat: GitHub OAuth Device Flow for one-click sign-in

The PR should be clean to merge now.