feat: flip napoln add default scope from global to project

[raiderrobert]

Implements #69.

See #69 for context.

What changed

• Added napoln.core.project module with is_inside_project, load_config_default_scope, and resolve_scope helpers.
• napoln add now defaults to project scope when run inside a recognized project (directory containing .git, .napoln, .claude, .agents, or .cursor).
• Added --global / -g flag to napoln add for explicit global installation opt-in.
• napoln add outside any project errors with a helpful hint suggesting --global.
• Bootstrap skill (napoln-manage) is now always installed globally, regardless of command scope.
• Default default_scope in new config.toml changed from "global" to "project".
• Updated existing tests to be explicit about scope where they relied on the old global default.

How to verify

# Inside a git repo or project with agent dirs
uv run napoln add ./my-skill          # installs into project scope

# Outside any project
uv run napoln add ./my-skill          # errors with hint to use --global
uv run napoln add ./my-skill --global # installs globally