IntelliJ plugin: restore sandbox/K2 verification path

[amichne]

Summary

Finish IntelliJ plugin hardening and verification for modern Kotlin/K2 sandbox paths.

Why

Packaging succeeds, but sandbox verification is still partially bypassed. That means the plugin can build without yet proving the compatibility path that matters for real IDE startup and release confidence.

Scope

• Add the required Kotlin plugin-mode compatibility metadata or equivalent configuration
• Re-enable buildSearchableOptions
• Verify sandbox startup on the intended IntelliJ/Kotlin versions
• Capture any remaining plugin verifier or packaging constraints in docs/tests

Done when

• backend-intellij passes the sandbox verification path without disabling buildSearchableOptions
• Runtime compatibility for the target IDE/Kotlin mode is proven, not assumed

References

• backend-intellij/build.gradle.kts
• backend-intellij/src/main/resources/META-INF/plugin.xml
• docs/remaining-work.md

[amichne]

Status update as of March 30, 2026:

• Add the required Kotlin plugin-mode compatibility metadata or equivalent configuration: not done yet.
• Re-enable buildSearchableOptions: not done yet; backend-intellij/build.gradle.kts still disables it on main.
• Verify sandbox startup on the intended IntelliJ and Kotlin versions: not done yet.
• Capture remaining plugin verifier or packaging constraints in docs and tests: partial; PR Add standalone analysis support and plugin verification coverage #10 adds plugin verifier and artifact checks in CI, but that does not restore the sandbox and K2 startup path.

Current read: verification coverage is improving in PR #10, but the specific sandbox and K2 compatibility work in this issue is still open.