Command contributions to a composite tool.

An addon contributes named subcommands to a host tool, so
`analysis command="lint ..."` reaches the contributing addon's handler.
Each contribution records the contributing addon so shutdown can retract
everything that addon added.

Shape: {tool-name {command-name {:handler fn :params {} :description str
                                 :addon addon-id}}}

Opaque capability registry.

An addon registers an implementation under a keyword key at startup; a
consumer looks it up without knowing which addon provides it:

  (register! :gs/struct-cmp my-cmp-fn)

  (if-let [f (get-extension :gs/struct-cmp)]
    (f a b)
    fallback)

Registration is atomic and idempotent — re-registering a key replaces it.

Schema-property extensions contributed to a host tool.

An addon widens an existing tool's input schema by registering extra
properties under that tool's name. Contributions merge, so several addons
may extend the same tool.

Dynamically registered tool definitions.

A tool definition is a map carrying at least a :name string and a
:handler fn. Registration is keyed by :name, last write wins.