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/commands/trekendsession.md

@@ -2,7 +2,7 @@
 name: trekendsession
 description: Mark the current session as complete and write session-state pointing at the next session. Helper for informal multi-session flows.
 argument-hint: "<next-brief-path> <next-label> | --help"
-model: sonnet
+model: opus
 ---
 
 # Voyage End-Session Local v1.0

plugins/voyage/commands/trekplan.md

@@ -501,7 +501,7 @@ ownership, hot files, and active branches that may affect planning."
 
 ### Launch for medium+ codebases (50+ files):
 
-**Convention Scanner** — use the `convention-scanner` plugin agent (model: "sonnet")
+**Convention Scanner** — use the `convention-scanner` plugin agent (model: "opus")
 for medium+ codebases only.
 Provide concrete examples from the codebase, not generic advice."
 
@@ -538,7 +538,7 @@ Common reasons for deep-dives:
 - A test pattern was identified but the test infrastructure needs more detail
 - A risk was flagged but the actual impact needs verification
 
-For each significant gap, spawn a targeted deep-dive agent (model: "sonnet",
+For each significant gap, spawn a targeted deep-dive agent (model: "opus",
 subagent_type: "Explore") with a narrow, specific brief.
 
 Launch up to 3 deep-dive agents in parallel. If no gaps exist, skip this phase

plugins/voyage/commands/trekresearch.md

@@ -291,7 +291,7 @@ other agents — the value of Gemini is independence.
 ### Launch rules
 
 - Launch ALL selected agents **in parallel** in a single message
-- Use model: "sonnet" for all sub-agents (the orchestrator runs on Opus)
+- Use model: "opus" for all sub-agents (the orchestrator runs on Opus)
 - Scale maxTurns by codebase size for local agents (same as trekplan):
   small = halved, medium/large = default
 - convention-scanner: medium+ codebases only (50+ files)
@@ -301,7 +301,7 @@ other agents — the value of Gemini is independence.
 Review all agent results. Identify knowledge gaps — areas where findings are
 thin, contradictory, or missing.
 
-For each significant gap, launch a targeted follow-up agent (model: "sonnet")
+For each significant gap, launch a targeted follow-up agent (model: "opus")
 with a narrow, specific brief. Maximum 2 follow-ups.
 
 If no gaps exist, skip: "Initial research sufficient — no follow-ups needed."