feat: support OpenCode v2 with safe sync tool commands

[iHildy]

Summary

Adds one package entrypoint for OpenCode v1 and v2. V1 keeps its existing server hooks and /sync-* commands. V2 registers opencode_sync as a tool, uses domain transforms for MCP, agent, model, and provider overrides, and shares the sync service with v1. README and docs/v2.md document the version requirements and remaining parity gaps.

This contains the complete work from PR #81 plus the focused safety fix in the contributor-branch fix PR. PR #81 should be superseded by this PR after review; this PR does not close it automatically.

Safety fix found during live verification

The original v2 /sync-status callback posted its result through session.synthetic. OpenCode treated that text as a new prompt, and the model attempted opencode_sync init. V2 now exposes sync operations through the opencode_sync tool only. The /sync-* commands remain on v1. V2 tool results rewrite v1-only slash-command hints to the equivalent tool arguments.

Verification

• bun run check and bun run build passed.
• bun test passed, 202 tests.
• Packed the branch and loaded it in an isolated OpenCode v2.0.13 profile. plugin list showed opencode-synced; command.list showed no sync-* commands.
• In a live v2 session, opencode_sync with { "command": "status" } returned visible tool content: opencode-synced is not configured. Run opencode_sync with {"command":"init"} to set it up. No configuration changed.
• The two-instance v1 model-driven test reached plugin startup but stopped before sync-init: OpenCode's default free model rejected API use, and an OpenAI OAuth retry failed token refresh with 401. The package's v1 entrypoint and hooks passed local tests; full v1 sync behavior remains unverified in this run.

Review notes

• V2 overrides outside mcp, agent, model, and provider warn and are not applied.
• V2 has no toast or session-idle API. Turso sync runs without idle gating.
• V2 override edits require host restart.
• GitHub's external-contributor approval gate initially held PR #81 CI. Its workflow was approved for execution after a read-only safety review.