fix(core): preserve provider identity in catalog updates

[kitlangton]

Why

A provider update could change provider.id while leaving the catalog entry indexed under its original ID. Listing the provider then advertised an ID that could not retrieve that entry. Agent, model, skill, and integration updates already preserve their keyed identities.

What Changes

Provider updates now restore the ID passed to update after the callback completes. Other field edits still apply.

Operation	Before	After
Update original, assigning ID renamed and name Updated	Stored under original, but reports ID renamed	Stored under original and reports ID original; name becomes Updated
Read using renamed	No entry	No entry: updating is not renaming

The regression covers creation, an existing provider, editor reads, public lookup/listing, and replay after reload.

Scope

One runtime assignment in Catalog.provider.update, matching the existing model-editor behavior. This does not redesign editor methods or make all returned catalog values immutable.

Verification

# packages/core
bun run test test/catalog.test.ts --test-name-pattern 'preserves provider identity'
bun run test test/catalog.test.ts test/agent.test.ts test/state.test.ts test/state-replay.test.ts
bun typecheck

# repository root
bunx prettier --check packages/core/src/catalog.ts packages/core/test/catalog.test.ts
git diff --check

The focused regression failed before the fix (original expected, renamed received). After the fix, all 59 focused tests pass, including the regression; Core typechecking and formatting pass. The normal pre-push workspace typecheck also passed.