fix: make generated JSON Schemas self-contained and strict-oneOf-safe

[joshmouch]

Fixes #302.

Both bugs are fixed generator-wide, plus one scope correction and a latent crash surfaced along the way.

Bug 1: empty {} oneOf branch

A union type alias's printed type text carries a leading | (type X =\n | A\n | B), so splitUnionType yielded an empty first element that fell through typeTextToSchema to {}. Every union alias's oneOf therefore led with a matches-anything branch, so every instance matched two or more branches and strict oneOf validation always failed.

While fixing this I hit a pre-existing infinite recursion the union path could already trigger: an inline object with a nested union field (e.g. side?: 'before' | 'after') contains a |, so it entered the union branch; splitUnionType correctly kept it as a single part (the | is nested inside {}), and the single-part early-return re-processed the identical string forever. The union branch now fires only for a genuine top-level union (two or more parts) and otherwise falls through to the inline-object / reference handling.

Bug 2: dangling $refs (broader than the issue scopes it)

One correction on scope: the dangling-ref problem is not unique to notifications.schema.json. Every generated file has them. Counts before this PR:

file	dangling $refs
state.schema.json	74
actions.schema.json	154
commands.schema.json	182
notifications.schema.json	95
errors.schema.json	103
total	608

And most of those (roughly 60-75% per file) are a second root cause the issue does not mention: the generator never walked enums. Any property typed as a bare const enum (SessionStatus, TurnState, ...) or an enum-member-literal discriminant (type: ActionType.AnnotationsEntryRemoved) hit the capitalized-identifier fallback in typeTextToSchema and was emitted as a $ref to a $def that was never defined, in any file. The remaining dangling refs were type aliases (URI, JsonPrimitive, ...) that only two of the five generators transitively collected.

The fix has three parts:

• Enum-member accesses inline as { "const": <member value> } (so type: ActionType.Foo becomes { "const": "root/agentsChanged" } rather than a dangling ref).
• A fixpoint resolveMissingDefs pass runs after each schema is built and backfills every referenced $def that is not yet present, resolving interfaces, type aliases (including URI = string), and whole enums (SessionStatus becomes { "type": "number", "enum": [1, 2, 8, 24, 32, 64] }, with its bitwise member values evaluated). It throws loudly if a referenced name resolves to none of those, so a future gap fails the build instead of silently emitting a dangling ref.
• null union members now render as { "type": "null" } instead of {}.

I chose transitive inline collection (option A from the issue) rather than cross-file $refs (option B), since every existing schema file is already self-contained and I did not want to introduce a new cross-document architecture.

Result

After this PR, across all five files: 0 dangling $refs (was 608) and 0 empty {} oneOf branches (was 30).

There was no test coverage for the schema generator and no CI gate that would have caught any of this, so I added scripts/generate-json-schema.test.ts (wired into npm test) asserting that no generated schema has an empty {} oneOf branch or a dangling $ref. It is dependency-free (no new packages) and fails if either bug is reintroduced.

npm test passes (290 tests, typecheck, lint, changelog + release-metadata gates). All five schema/*.json files are regenerated in this PR.