fix(plugin): memcommit session resolution and legacy commit compatibility

[LittleLory]

Summary

This PR fixes memcommit for the OpenCode memory plugin example.

It resolves two related issues:

• the plugin was reading the wrong OpenCode tool-context field when trying to resolve the current session mapping
• the plugin assumed the newer background commit/task API was always available, which broke compatibility with older OpenViking servers

Changes

• use context.sessionID instead of context.session?.id when resolving the current OpenCode session
• fix the same session-context issue in both memcommit and memsearch
• detect whether the OpenViking server supports background commit tasks
• fall back to synchronous POST /api/v1/sessions/{id}/commit when the server does not provide /api/v1/tasks
• keep the background task flow for newer servers that support it

Why

The OpenCode plugin ToolContext exposes sessionID, not session.id.

Also, the plugin example currently needs to work against both:

• newer OpenViking servers with background commit task APIs
• older OpenViking servers that only support synchronous session commit

Without this compatibility handling, memcommit can fail even when the session mapping itself is valid.

Notes

This change does not yet alter the user-facing memcommit waiting behavior. It focuses on correctness and compatibility first.

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