feat(effect-zod): transform support + walk memoization + flattened checks

[kitlangton]

Summary

Fourth walker extension in the Zod-from-Schema pipeline. Adds transform support and two refinements to the existing code path that were worth doing together.

Transform support (Schema.decodeTo / Schema.transform)

When an AST has an `encoding` chain, the walker now threads each link's decode getter through a Zod `.transform()`. This enables migrating any Zod schema that uses `.transform()` or `.preprocess(...).transform()` — notably `agent.ts` and `permission.ts#Info`.

Crucial nuance: `Schema.Class` uses `Declaration` + encoding internally to construct class instances. For Zod consumers we want plain objects, not class instances, so Declaration nodes fall through to `body()` and skip encoding. Regression-guarded by a dedicated test.

Memoized `walk()`

AST nodes are immutable and frequently shared across schemas (a single `Schema.Class` embedded in multiple parents is common). Added a module-scoped `WeakMap` cache keyed by AST identity:

• Same AST → same derived Zod instance.
• Nested reuse shares children by reference.
• `WeakMap` auto-GCs with the AST.

Flattened check refinements

Previously N checks produced N stacked `.superRefine` wrappers, each traversed per parse. `applyChecks()` now collects all filters from `checks` + `FilterGroup` into one linear list and runs them inside a single `.superRefine` — one Zod wrapper regardless of count, still all checks fire and all error messages are emitted.

Tests (9 new)

Transform:

• Number → Duration-ish (mirrors `DurationFromSeconds` in `account.ts`)
• String → Number via `parseInt`
• Transform inside a struct field
• Chained `decodeTo` exercising the `encoded()` reduce
• Schema.Class guardrail — parsed output is a plain object, not an instance

Memoization:

• `walk()` memoizes by AST identity
• Nested reuse shares cached children by reference

Flattened checks:

• Multiple independent checks all fire in one refinement layer
• `FilterGroup` flattens alongside its siblings

Correctness

• All 1950 opencode tests pass, zero failures
• Typecheck clean
• SDK byte-identical (confirmed via `./packages/sdk/js/script/build.ts`)

Performance

• Transform / check work moved from per-parse layer allocation to per-walk setup
• Shared AST subtrees cached by identity, eliminating duplicate walks when multiple schemas embed the same class
• `EMPTY_PARSE_OPTIONS` hoisted to module scope (no per-parse allocation)

Simplify pass

Ran the /simplify workflow. Aggregated findings from three parallel reviews and applied the worthwhile ones: inlined `runDecode` into `decode` (dropped ast-threading for error messages, collapsed double cast), consolidated walk-level intermediate consts per the style guide, added the memoization + check-flattening above. Pre-existing style nits and out-of-scope perf ideas deferred to future PRs.

Follow-ups unlocked

• `config/agent.ts` — top-level `.catchall` + `.transform`
• `config/permission.ts#Info` — `.preprocess` + `.transform` + key-order hack