fix(tool): register lsp tool when LSP is configured, not behind experimental flag

[YOMXXX]

Summary

Fixes #2017. The lsp tool never appears in the agent tool list even when LSP servers are configured (e.g. lsp.rust-analyzer).

Root cause: packages/opencode/src/tool/registry.ts gated the lsp tool behind Flag.MIMOCODE_EXPERIMENTAL_LSP_TOOL, which defaults OFF. The tool was therefore never registered for normal users regardless of LSP configuration or server-launch timing.

Fix: register the lsp tool when LSP is configured — lspEnabled = Boolean(cfg.lsp) || Flag.MIMOCODE_EXPERIMENTAL_LSP_TOOL:

• config.lsp truthy (true or a server record) → lsp tool registered.
• config.lsp falsy (default) → not registered (no token overhead for non-LSP users).
• The experimental flag is retained as a force-enable override (backward compatible).

The lsp tool itself already handles "no server for this file type" at runtime via lsp.hasClients, so no change to the tool's execute path.

Test Plan

• New tests: registry.ids() contains lsp with config: { lsp: true }; does NOT contain lsp with config: { lsp: false }.
• bun test test/tool/registry.test.ts — 7 pass.
• bun typecheck — clean.