fix: flatten plugin structure for hook path resolution

Moved .claude-plugin/, hooks/, and skills/ from
plugins/claude-git-native-issue/ to the repo root.

Claude Code resolves CLAUDE_PLUGIN_ROOT and hook paths relative
to the repo root. The nested structure caused the UserPromptSubmit
hook to silently fail because the resolved path didn't exist.

Matches the flat structure used by official plugins (e.g. context7).

feat: initial plugin — autonomous task management via git-native-issue

Claude Code plugin that replaces internal TaskCreate/TaskUpdate/TaskList
with git-native-issue commands. Three-layer activation: UserPromptSubmit
hook for ambient context injection, ambient skill with autonomous decision
rules and full CLI reference, and 6 user-invocable slash commands
(setup, create, issues, show, close, sync).

Autonomous behavior: auto-create issues for multi-step work, auto-update
with progress labels/comments, auto-close on task completion, and
auto-sync with platform provider (GitHub/GitLab/Gitea) after mutations.
Provider detected from git remote URL and cached in git config.