refactor(actions): make Context a type-keyed extensions map

[passcod]

Summary

Replaces the two-slot `Context<TopArgs, SubArgs>` with a type-keyed extensions map, axum/React-style. Each subcommand level provides its own args struct into the context; leaf handlers extract whichever ancestors they need via `ctx.require::()`.

Motivation

The old `Context<A, B>` could only carry two args shapes (immediate parent + self), so a handler nested more than two deep would lose access to its grandparent's args (e.g. `Args` from inside a tamanu leaf). Deep features like the WIP `tamanu logs` and `reload` would either have to thread args through manually or duplicate state.

What changed

• `context.rs`: `Context` is now a `HashMap<TypeId, Arc<dyn Any + Send + Sync>>` with `provide` / `get` / `require` methods. It's `Clone` (cheap — only Arc clones).
• `subcommands!` macro: takes a `(args, ctx) -> (Action, Context)` prep closure; emits a uniform `async fn run(args, ctx)` dispatcher. Each level inserts its own args into ctx before dispatching to children.
• All 25+ leaf and branch handlers migrated to the new `async fn run(args: Self, ctx: Context)` signature. Handlers access ancestor args via `ctx.require::()`.

Tradeoff

Compile-time "you definitely have these two levels" guarantee → runtime "panic if you forgot to provide". Mitigated by the macro inserting symmetrically and 5 unit tests on the Context primitives. Worst case is `bestool foo bar` panicking with a clear `required value of type `X` not provided` if I missed a provide() somewhere — which we'd catch on the first dev run.