fix: render tool schemas with root-level anyOf/oneOf composition

A tool's `inputSchema` may carry composition keywords at its root — the
2026-07-28 revision makes this explicit — but both form builders enumerate
the root's `properties` and nothing else. A schema keeping its fields on
`allOf`/`oneOf`/`anyOf` branches therefore rendered no controls at all: no
branch picker, no fields, not even the raw-JSON fallback a union-typed
*property* falls back to, so the tool could only be called with empty
arguments.

Adds `core/json/rootUnion.ts`, one flattening shared by all three clients so
they cannot drift on which schemas they can render:

- `allOf` is merged unconditionally (conjunctive — there is no choice to
  present), a root `oneOf`/`anyOf` is returned as branches.
- Web `SchemaForm` renders a Variant picker above the fields and prunes the
  outgoing branch's values on a switch, since they are no longer visible and
  would describe a shape the call is not making.
- TUI `schemaToForm` gives each branch its own section with optional fields —
  ink-form is static, and only one alternative applies to a call.
- `convertToolParameters` looks through the branches, so a CLI `--tool-arg`
  is typed by the schema that declares it rather than sent as a string.

Required-field gating asks whether *any* branch is satisfied, which is
selection-independent and never blocks arguments the schema accepts. A
`const` is seeded like a `default` and rendered read-only: it admits exactly
one value, so anything else the user could type produces a rejected call.

Adds a `root-union-schemas-http.json` showcase server.

Closes #2123

Signed-off-by: cliffhall <cliff@futurescale.com>

AGENTS.md

@@ -158,6 +158,25 @@ v2/main/
 │   │                                   #   and TUI schemaToForm — since each dispatches on
 │   │                                   #   a single `type` string and would otherwise miss
 │   │                                   #   a nullable field entirely — #1928/#2015;
+│   │                                   #   rootUnion.ts: flattens the COMPOSITION keywords a
+│   │                                   #   tool's inputSchema may carry at its ROOT — `allOf`
+│   │                                   #   merged unconditionally (conjunctive, so there is
+│   │                                   #   no choice to present), a root `oneOf`/`anyOf`
+│   │                                   #   returned as the BRANCHES a picker chooses between.
+│   │                                   #   Legal since 2026-07-28 and rendered as an EMPTY
+│   │                                   #   FORM before — no picker, no fields, not even the
+│   │                                   #   raw-JSON fallback a union-typed *property* gets,
+│   │                                   #   so the tool could only be called with empty
+│   │                                   #   arguments. Read by all three clients: web
+│   │                                   #   SchemaForm (the Variant picker + the branch-change
+│   │                                   #   value pruning), TUI schemaToForm (a section per
+│   │                                   #   branch, its fields forced OPTIONAL since only one
+│   │                                   #   alternative applies), and convertToolParameters
+│   │                                   #   (which branch's schema types a CLI --tool-arg).
+│   │                                   #   Declines a union whose members are not ALL
+│   │                                   #   field-carrying objects rather than offering a
+│   │                                   #   picker with options that render nothing, and does
+│   │                                   #   not interpret `not` at all — #2123;
 │   │                                   #   schemaLint.ts: tool-schema PORTABILITY lint —
 │   │                                   #   constructs that are legal JSON Schema and are
 │   │                                   #   refused or mishandled by real MCP clients (a bare