Frugal model overrides

Manage the per-project model mapping for frugal's agents. Defaults live in each agent's frontmatter (agents/*.md under the plugin root); overrides live in the project's .claude/routing-overrides.md, which the routing policy reads first. Overrides work because the Agent tool accepts a model parameter per spawn - no plugin files are ever edited.

Valid agents: scout, extractor, mechanic, builder, sage. Valid models: haiku, sonnet, opus, fable.

No arguments: show the mapping

1. Read each agent's default from the model: frontmatter line in <plugin-root>/agents/*.md (plugin root is two directories up from this skill's base directory).
2. Read the "Model overrides" section of .claude/routing-overrides.md if it exists.
3. Show one table: agent, default, override (or "-"), effective model. Nothing else.

Arguments like scout=sonnet builder=opus

1. Validate every pair against the lists above. Invalid agent or model: reject with the valid options, change nothing.

2. Create or update the managed section in .claude/routing-overrides.md (create the file if missing, keep any unmanaged content above it intact):

   ## Model overrides (managed by /frugal:models)
   
   | Agent | Model |
   |---|---|
   | scout | sonnet |
   
   When spawning a frugal agent listed above, pass its listed model as the
   Agent tool's `model` parameter.

3. Merge with existing overrides: new pairs win, unmentioned pairs stay.
4. Setting an agent to its default removes its row. Empty table: remove the whole section.
5. Confirm with the resulting mapping table and apply it immediately to any spawns later in this session.

Argument reset

Remove the managed section. If the file is then empty, delete the file. Show the default mapping.