Support multi-agent repo initialization (multiple --tool targets)

Problem

Currently swamp repo init and swamp repo upgrade support a single --tool target. The repo marker records one tool, and switching tools requires upgrade -t <tool> or init --force -t <tool>.

In practice, users work with multiple AI coding agents (e.g., Claude Code, Kiro CLI, OpenCode) against the same swamp repository. The .swamp/ data layer (models, workflows, vaults, data) is already tool-agnostic — only the agent scaffolding (instructions files, skills directories, settings, audit hooks) is tool-specific.

Current workaround

Run swamp repo upgrade -t kiro to switch, then manually preserve the previous tool's config (e.g., keep .claude/ around). This works because the file paths don't actually conflict between tools, but the marker only tracks one tool and upgrade only writes scaffolding for the target tool.

Proposed solution

Allow swamp repo init and swamp repo upgrade to accept multiple --tool values, or add an --also / --add-tool flag:

# Initialize for multiple agents at once
swamp repo init --tool claude --tool kiro

# Add a tool to an existing repo
swamp repo upgrade --add-tool kiro

The marker file would track an array of tools. Each tool's scaffolding (instructions, skills, settings, hooks) would be written independently since they already use non-overlapping paths.

Why this matters

• Users increasingly switch between agents depending on the task
• The swamp data layer is already shared — only the agent scaffolding needs multi-tool awareness
• Current UX forces users to choose one tool or manually manage the others