feat(usage): OpenCode z.ai coding-plan usage source #1104

Motivation

z.ai GLM Coding Plan users (via OpenCode) have no usage surface. Research (spec §Readers/opencode:zai) verified live: GET https://api.z.ai/api/monitor/usage/quota/limit with the raw opencode zai-coding-plan key as Authorization (no Bearer — Bearer 401s), key from ~/.local/share/opencode/auth.json.

Spec: docs/superpowers/specs/2026-09-20-provider-usage-enablement-design.md §Readers/opencode:zai.

Intended behavior

• Settings: OpenCode usage-source dropdown becomes live (none | zai), greyed with a hint when OpenCode is disabled or no credential exists.
• Registry source opencode:zai active when OpenCode enabled AND source = zai.
• Response mapping: classify CREDIT_LIMIT windows by unit (3 = 5-hour, 6 = weekly; never positional), nextResetTime epoch ms → reset, plan = data.level, both rows all-models; HTTP 200 with success:false inner envelope ⇒ error row; unclassifiable layout ⇒ error, not a guess.
• Security: single read-only request; key only in memory + header; opencode auth.json read-only.

Phase 3 of the spec. Depends on Phase 1 (registry + settings dropdown plumbing).

Acceptance criteria

• Reader mapping unit tested with order-shuffled fixtures, inner-failure envelope, unclassifiable layout.
• Dropdown state logic tested (disabled cases + credential absence).
• z.ai row renders in modal rail/detail; exhaustion integration works (all-models scope).
• Live-verified response fixture captured 2026-09-20 committed (no key material).