test(opencode): remove obsolete HTTP API exerciser

[kitlangton]

What

Remove the standalone HTTP API exercise harness from the legacy packages/opencode package.

The harness mixed legacy and V2 compatibility routes in one scenario table, was not wired into CI, and no longer provided a green validation target:

• 31 current OpenAPI routes had no scenario
• 4 scenarios targeted routes no longer present in the contract
• 12 Effect-mode scenarios failed
• the DSL retained unused state and terminal operations

How

• Delete test/server/httpapi-exercise and its route scenario DSL.
• Delete the one-line script/httpapi-exercise.ts launcher.
• Remove the unused test:httpapi package script.

Scope

This does not replace the harness with a new V2 integration suite. Focused server and middleware tests remain in place; any future broad V2 API coverage should live with the V2 package set rather than the legacy compatibility package.

Testing

• bun typecheck from packages/opencode
• repository pre-push hook: bun turbo typecheck --concurrency=3 (31 packages passed)
• bunx prettier --check package.json
• verified no remaining test:httpapi or httpapi-exercise references