feat: restore experimental flag for websearch/codesearch tools

[ben-vargas]

Re-enables the OPENCODE_EXPERIMENTAL_EXA flag to allow non-zen users to opt-in to websearch and codesearch tools.

Context

Commit d95f724 enabled these tools for everyone, then 9c237f0 restricted them to zen users only while figuring out enterprise opt-out.

This PR provides an interim solution: zen users get the tools by default, while other users can opt-in via OPENCODE_EXPERIMENTAL_EXA=true (or the general OPENCODE_EXPERIMENTAL=true flag).

Changes

• Restore OPENCODE_EXPERIMENTAL_EXA flag in flag.ts
• Update registry.ts filter to allow tools when providerID === "opencode" || Flag.OPENCODE_EXPERIMENTAL_EXA

This matches the original experimental implementation pattern while preserving the zen-first behavior.