fix: keep v2 sync status read-only

[iHildy]

Why

In PR #81, OpenCode v2 treats session.synthetic output from /sync-status as a new prompt. In a packed-plugin smoke test, the model then called opencode_sync with command: init without a user request to initialize. A separate check showed resume: false prevents that follow-up but leaves the result outside the visible transcript.

Change

• Keep all opencode_sync tool operations available on v2, including status, init, and link.
• Do not register /sync-* slash commands on v2 until there is a safe, visible command result channel. V1 slash commands remain available.
• Rewrite v1 slash-command hints in v2 tool results to show the equivalent opencode_sync arguments.
• Update README and v2 docs to describe the difference.

Verification

• bun run check, bun test (202 pass), bun run build.
• Packed the branch and loaded it in an isolated OpenCode v2.0.13 profile. The plugin appears in plugin list, while command.list contains no sync-* commands.
• In a live v2 session, opencode_sync with { "command": "status" } returned visible tool content and made no configuration change.
• The two-instance v1 model-driven test reached plugin startup but stopped before sync-init because the available free model rejected API use; an OpenAI OAuth retry failed token refresh with 401. No sync logic was exercised in those runs.

This PR is based on the exact head of PR #81, so the diff against that branch is limited to this safety fix.