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

plugin/scripts/sfs-sync.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+# Sync the current project if it is an sfs mount (has a .sfs settings file).
+# Fast no-op otherwise, so this hook is safe on every turn in every project.
+#
+# Runs blocking on UserPromptSubmit (fresh files before Claude reads them)
+# and async on Stop (push edits out without delaying the turn).
+cd "${CLAUDE_PROJECT_DIR:-.}" || exit 0
+[ -f .sfs ] || exit 0
+command -v sfs >/dev/null 2>&1 || exit 0
+sfs sync . >/dev/null 2>&1 || true