fix: sync plural OpenCode config directories

[iHildy]

Summary

• sync OpenCode's canonical plural agents/, commands/, modes/, plugins/, and tools/ config directories by default
• retain every legacy singular directory so existing repositories remain compatible
• keep skills/ under its existing setting and filter default-directory overlaps from extraConfigPaths so each local and repository path is planned once

Verification

• bun run check
• bun test — 88 tests passed
• bun run build
• feature-focused two-home regression: plural agent, command, mode, plugin, and tool sentinels copied from machine A through the sync repository to machine B
• full isolated GitHub E2E passed: machine A init/push, machine B link, a second machine A push, and machine B pull/replication; the active source tree stayed unchanged and the ephemeral repository was deleted

The E2E harness emitted its known local-materialization warning: machine B's live AGENTS.md did not refresh after the final pull, while its isolated sync clone did contain the second sentinel, confirming repository replication. Loopback servers also emitted the expected unsecured-server warning in the isolated sandbox.

Closes #67