feat(plugin): add session request hook

[rekram1-node]

Summary

• add Effect and Promise ctx.session.hook("request", ...) APIs for mutable model headers and JSON bodies
• transform native HTTP and final WebSocket request payloads before authentication and signing
• bridge cached AI SDK models with request-local transforms while preserving concurrent Session isolation
• apply Session request hooks to normal steps, title generation, and automatic/manual compaction

Simplifications

• share the AI request JSON type with the plugin contract instead of duplicating it
• preserve the original WebSocket encoding path when no hook is registered
• resolve only the effective agent for manual compaction instead of loading full instruction/plugin context
• preserve upstream compaction cost tracking while carrying hook identity

Tests

• cd packages/plugin && bun typecheck
• cd packages/ai && bun typecheck && bun test (331 passed, 29 skipped)
• cd packages/core && bun typecheck
• focused Core suites (156 passed)
• full Core suite (1338 passed, 6 skipped, 1 environment-only failure from user plugins in ~/.config/opencode/plugin)