feat: web viewer, project-local config, selective sync, Claude Code plugin

- sfs-web (cmd/sfs-web, internal/webapp): read-only Obsidian-style web UI
  serving a local folder (default) or an sfs remote; markdown rendering
  with [[wikilinks]], task lists and tables, file downloads with ETags,
  per-file provenance from the journals; added to goreleaser builds
- .sfs project file (internal/config): per-folder volume/remote/include
  settings that travel with the folder, win over the global registry, and
  never sync; daemon picks up edits live
- .sfsignore + include lists (internal/syncer): gitignore-style selective
  sync with ! re-includes, applied symmetrically in scan and materialize;
  newly ignored files stop syncing without being deleted anywhere
- Claude Code plugin (plugin/, .claude-plugin/): sfs skill, /sfs:mount and
  /sfs:status commands, turn-boundary sync hooks (blocking pull on prompt,
  async push on stop); installable via the repo's marketplace manifest
- CLAUDE.md and .claude project settings for Claude Code development

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HHEUaYfFHhmDvqLYw74Ehz

go.sum

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