Add launchers.<cmd>.sharedDirs: share a state-dir directory between launchers

Launchers with different ids share nothing, which is right for logins but
not for Claude's projects (sessions and memory) or Codex's memories. A
shared dir is a managed symlink to the other launcher's copy, created on
demand so the link holds whichever launcher starts first, and removed with
the declaration while the data stays.

Add claude.mcpServers: MCP servers loaded with --mcp-config

The flag is variadic, so it goes first and --settings always follows it;
otherwise a prompt after it would be read as another config file. Logins
stay in the account.

Describe --model in the companions skill

The Claude Code call now says how to pick a model: an alias (fable, opus,
haiku) for the latest of a family or a full name to pin a version. The
companions check greps the generated skill for the flag.

Add claude.plugins: plugin directories loaded with --plugin-dir

Add packages: tools on every launcher's PATH

Add companions: every launcher of an environment callable from its sessions

Export AGENT_ENV_LAUNCHER, the launcher's command name

Add launchers with a state dir per login

Add Claude Code and Codex agent modules

Add core module: instruction fragments and skill closure