fix(core): gate fork_session_for_plugin behind the opencode-plugin-host feature

[1688mengdie]

Summary

Fixes #2570

Gate CoreProductAgentRuntime::fork_session_for_plugin (src/crates/assembly/core/src/product_runtime.rs:614) behind #[cfg(feature = "opencode-plugin-host")].

The function's only consumer is the plugin host HTTP routes implementation, which the crate compiles under the opencode-plugin-host feature gate in lib.rs. Test builds that do not enable the feature — notably the sdk-host test compilation unit in the upstream CI CLI Tests jobs on ubuntu/macos — therefore reported the function as dead code (see issue #2570 for the run evidence with file:line and rule names).

Type and Areas

Type: bug fix (build warnings cleanup)

Areas: Rust core

Motivation / Impact

No direct user-facing change. The plugin-host build keeps the function available for its single consumer; feature-less test builds stop emitting the dead_code warning. This also makes the declaration's scope match the existing feature gate around its consumer.

Verification

• cargo check --locked -p bitfun-core on Windows: exit 0, no dead_code warning for fork_session_for_plugin.
• cargo check --locked -p bitfun-core --features opencode-plugin-host on Windows: exit 0 (the gated symbol is still available to the plugin-host build).
• The exact failing configuration (sdk-host test profile on ubuntu/macos) depends on the CI ubuntu/macos runners: the CLI Tests jobs should confirm the warning is gone there (n/a locally — no Linux/macOS machine available; the platform matrix in CI is exactly the place this gets verified).

Reviewer Notes

• Single file, +1 attribute line, zero deletions, zero behavior change.
• No #[allow] suppression is introduced.
• AI-assisted change, lightly tested (cargo check with and without the feature on Windows; the ubuntu/macos test-profile surface is verified by CI).

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable. (n/a — no user-facing change)