fix(opencode): resolve command agent ids to registered opencode agents (#2477)

The `.opencode/commands/*.md` frontmatter referenced agents with the Claude
Code plugin namespace (`agent: everything-claude-code:<name>`), but ECC's
opencode integration registers its agents unscoped in `opencode.json`'s
`agent` map (`code-reviewer`, `planner`, ...), and that file's own `command`
section already references them unscoped. The `everything-claude-code:` scope
resolves under no opencode config (the opencode plugin package is
`ecc-universal`, and inline-config agents are bare), so subtask commands like
`/code-review` hard-fail with `Agent not found: everything-claude-code:code-reviewer`.
Non-subtask commands fall back to the default agent and appear to work — which
is why only some commands failed.

Strip the `everything-claude-code:` prefix from all 30 command frontmatter
agent ids so they match the registered agents, fix the MIGRATION.md example,
and replace the test that enforced the broken scoped invariant with one that
asserts each command agent id is a registered opencode agent (fails on the old
scoped ids, passes on the fix).

Fixes #2477

.opencode/commands/orchestrate.md

@@ -1,6 +1,6 @@
 ---
 description: Orchestrate multiple agents for complex tasks
-agent: everything-claude-code:planner
+agent: planner
 subtask: true
 ---