feat: skill builder UI, daemon lifecycle, update notifications

[initializ-mk]

Summary

• Daemon Agent Lifecycle: Agents now run as forge serve daemon processes via exec.Command instead of in-process goroutines — agents survive UI shutdown and are auto-detected on restart via .forge/serve.json + TCP probe. Scanner is the single source of truth (removed MergeState, AgentStartFunc, External field).
• Auth & Channels: Agents start with bearer token auth enabled (no more --no-auth); configured channels (Slack/Telegram) passed via --with flag automatically.
• Skill Builder UI: Interactive LLM-powered skill creation wizard with real-time streaming chat, YAML frontmatter validation, and one-click save to agent config.
• Update Notifications: Dashboard checks GitHub Releases API (30-min cache, semver comparison) and shows animated "Update Available" banner when a newer version exists.
• Version Display: Sidebar footer shows current Forge version and link to useforge.ai.
• Install Script: One-line curl | bash installer/upgrader at repo root.
• Code Review Skills: diff, file, standards, and github review skills.
• Codegen Skills: React and HTML code generation embedded skills.
• A2A Auth: Bearer token authentication for A2A server.
• Documentation: Updated dashboard docs with skill builder section, installation docs with install script.

Test plan

• cd forge-ui && go test ./... — all tests pass
• cd forge-cli && go test ./... — all tests pass
• Start UI → start an agent → verify it shows as running with auth enabled
• Stop UI (Ctrl+C) → verify agent is still running
• Restart UI → verify agent shows as running (detected by scanner)
• Stop agent from UI → verify it stops
• Verify agent starts with channels when configured in forge.yaml
• Skill Builder: open chat, generate a skill, validate, save
• Update banner appears when running an older version