chore(voyage): pin all sub-agents to Opus permanently (operator request)

Flip model: sonnet → model: opus across 20 agent files, 4 prose references
in commands (trekplan, trekresearch), trekendsession command frontmatter,
and CLAUDE.md tables. Aligns CLAUDE.md premium-profile row to actual
premium.yaml content (all-opus, which has been the case since v4.1.0 but
the doc was drift). Companion to VOYAGE_PROFILE=premium env-var (set in
~/.zshenv same day) — env-var governs orchestrator phase model; this
commit governs sub-agent models which are frontmatter-pinned and not
reachable by the profile resolver.

npm test: 516 pass, 0 fail, 2 skipped (unchanged from baseline).

Operator rationale: complete Opus coverage across all Voyage activity,
including the 20 sub-agents that the profile system does not control
(architecture-mapper, task-finder, plan-critic, scope-guardian,
brief-reviewer, code-correctness-reviewer, brief-conformance-reviewer,
review-coordinator, session-decomposer, plus the 6 researcher agents,
plus the 5 codebase-analysis agents).

Cost implication: sub-agent runs ~5x more expensive vs sonnet. Accepted.

plugins/voyage/agents/code-correctness-reviewer.md

@@ -6,7 +6,7 @@ description: |
   cross-file regressions, test coverage gaps, placeholder code, security
   surface, hidden dependencies. Cites file:line for every finding. Never
   praises.
-model: sonnet
+model: opus
 color: red
 tools: ["Read", "Glob", "Grep"]
 ---