feat(gate): 四階段模型閘門改為指定模型優先、能力標籤備援，並以 sdlc-gate 上鎖

What：plan、analyze、implement、maintain 四個 SKILL.md 的第一步從單純的能力標籤檢查，改為「模型閘門與階段上鎖」三段流程：先以 jsc-cli/tools/model-config.sh get {stage} 解析該階段的指定模型鏈，鏈上第一個當前 CLI 可用的模型即為必用模型；沒有設定指定模型時才退回 jsc-cli:models 的能力標籤檢查。閘門通過後執行 jsc-hooks/hooks/sdlc-gate.sh lock {stage} {current-model} 寫入鎖檔，並以 sdlc-gate.sh report 驗證。

Why：原本只驗能力標籤，同一標籤下模型可任意替換，階段中途換模型也無人攔阻，導致產出品質與可重現性不穩定。改為指定模型鏈優先可讓團隊明確指派每階段用哪個模型，標籤備援則保留未設定時的彈性；上鎖後由 hook 在每次 prompt 強制檢查，階段內換模型會被擋下。

How：每個技能的步驟一拆成三小步（解析模型鏈、驗證當前模型、上鎖並驗證鎖檔），各小步附完成條件；maintain 另加第四小步，明確規定只在換階段時重跑閘門，維護階段內跨專案不重複驗證。frontmatter description 同步更新。鎖定自 plan 起手生效，直到下一階段（如 analyze）的閘門重新上鎖前，強制沿用同一模型。

Who：jsc-sdlc 的 plan／analyze／implement／maintain 四個技能，及依賴其閘門行為的 jsc-hooks sdlc-gate hook。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skills/plan/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: plan
-description: SDLC planning stage. Gate on model capability tags, pick or create a plan from PLAN_CONTENTS, then run a decision tree until goal, scope, and feasibility reach consensus. Produce user stories into wiki page PLAN_{HASH}. Logic only - never write code or modify files. Use when the user wants to start or refine a plan; not for analysis or implementation.
+description: SDLC planning stage. Gate on the designated model (model-config) or capability tags, lock the model for the stage via sdlc-gate, pick or create a plan from PLAN_CONTENTS, then run a decision tree until goal, scope, and feasibility reach consensus. Produce user stories into wiki page PLAN_{HASH}. Logic only - never write code or modify files. Use when the user wants to start or refine a plan; not for analysis or implementation.
 ---
 
 # plan
@@ -13,7 +13,10 @@ All wiki reads and writes go through `jsc-gitea:wiki`.
 
 ## Steps
 
-1. **Model capability gate**: check the capability tags from `jsc-cli:models` and confirm the current model qualifies for planning. If not, block the flow and tell the user which model to switch to.
+1. **Model gate and stage lock**:
+   1. Resolve the plan stage's designated model chain: run `jsc-cli/tools/model-config.sh get plan`. If it prints a chain, the required model is the first entry of the chain the current CLI can use (fallbacks apply in chain order). If it prints nothing, fall back to the capability-tag check via `jsc-cli:models` (planning requires the reasoning-high tag).
+   2. If the current model is not the required model, or fails the tag check, block the flow and tell the user which model to switch to. Completion condition: the current model satisfies the gate.
+   3. Lock the stage: run `jsc-hooks/hooks/sdlc-gate.sh lock plan {current-model}`. From now until the next SDLC stage's gate runs, the sdlc-gate hook enforces this model on every prompt; switching models mid-stage gets blocked. Completion condition: the lock file is written, verified with `sdlc-gate.sh report`.
 2. Read `PLAN_CONTENTS` via `jsc-gitea:wiki` and list the plans whose status is the literal 「未分析」 (not analyzed), with names and HASH.
 3. Let the user choose per `jsc-ask:ask` rules: **extend an existing plan** (list the not-analyzed plans as options) or **create a new plan**. State the impact scope on every option.
 4. Question via the `jsc-ask:ask` decision tree until no doubt remains on all three items; keep asking while consensus is missing: