[codex] Add opt-in struct tag field resolution

[myzie]

Summary

Adds opt-in struct tag based field resolution with expr.WithStructTags(...) and the WithFieldTags(...) alias. When configured, struct fields resolve by tag names in priority order before falling back to Go exported field names, while default behavior remains unchanged.

Details

• Supports comma-delimited tag names such as json:"email,omitempty".
• Implements strict per-field precedence, expr:"-" hiding, tag-specific "-" behavior, and ambiguity errors for duplicate resolved names.
• Wires tag-aware lookup through identifiers, selectors, selector-chain fast paths, callable struct fields, and suggestion hints.
• Caches tag field plans per compiled program and struct type to avoid repeated reflect.VisibleFields scans.
• Updates reference docs, examples, README, and LLM reference.

Performance

Focused benchmarks showed uncached tagged lookup was measurable, so this PR includes a cache:

• Nested tagged selector: from ~630 ns/op and 14 allocs to ~76 ns/op and 0 allocs.
• 32-field tagged struct: from ~9.4 us/op and 91 allocs to ~144 ns/op and 1 alloc.

Validation

• go test ./...
• go test -run '^$' -bench 'Benchmark_structTags' -benchmem -count=3
• git diff --check