refactor(core): establish plugin runtime boundary

[limityan]

Summary

• add plugin runtime boundary contracts in runtime-ports, including availability facts, dispatch/response envelopes, disabled binding, projection-only binding, and fail-closed dispatch behavior before a Host exists
• carry non-executable plugin runtime availability through Product Assembly and Agent Runtime while rejecting any real PluginRuntimeBinding::Client until Host gates are registered
• move LSP plugin runtime target facts and pure command placeholder resolution from services-core to core-types, leaving services-core as registry/current-target owner
• update the decomposition plan/completed docs with the UI Extension product-shape matrix and a separate future Plugin Runtime Host stage

Risk and compatibility

• no real plugin Host, ecosystem adapter, JS/TS runtime, worker, subprocess, package discovery, or external UI execution is enabled by this PR
• Agent Runtime SDK exposes plugin runtime availability facts only; client binding, dispatch envelopes, and concrete plugin clients are not re-exported through the SDK facade
• Product Assembly and Agent Runtime Builder both reject real client bindings, including clients that misreport projection-only availability
• no Cargo manifest or third-party dependency changes
• existing services-core LSP callers keep compatibility re-exports for runtime target and placeholder resolver names

Verification

• pnpm run fmt:rs
• node --test scripts/check-core-boundaries.test.mjs
• node scripts/check-core-boundaries.mjs
• pnpm run check:repo-hygiene
• cargo metadata --no-deps --format-version 1
• cargo test -p bitfun-runtime-ports
• cargo test -p bitfun-core-types
• cargo test -p bitfun-product-capabilities
• cargo test -p bitfun-agent-runtime
• cargo test -p bitfun-services-core --test lsp_plugin_registry_contracts
• cargo check -p bitfun-core --no-default-features
• cargo check -p bitfun-core --features product-full
• cargo check --workspace with local src/mobile-web/dist directory present for the desktop build-script precondition
• git diff --check