docs(copilot): add model selection reference

copilot/model-guide.md

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+# Model selection guide (Copilot CLI)
+
+Reference for picking a model/agent per plan phase. Referenced from
+`instructions/workflow.instructions.md` rather than inlined there, so this table can be
+updated independently whenever the available model lineup or pricing changes.
+
+Update this table directly when the model lineup changes — no need to touch the workflow
+instructions to do so.
+
+| Model | When to use it | 💡 Smartness | ⚡ Speed | 💲 Cost |
+| --- | --- | --- | --- | --- |
+| _(fill in current lineup here)_ | | | | |
+
+## Rough guidance
+
+- **Hardest, most open-ended tasks** (complex refactoring, ambiguous debugging, deep
+  architectural analysis, long multi-step reasoning): pick the highest-smartness model
+  available, accept lower speed/higher cost.
+- **Routine, well-specified work** (mechanical edits, bulk reads/exploration, formatting,
+  straightforward test/lint fixes): pick a lower-cost, faster model — this is most of a
+  typical phase's work.
+- **PR-feedback loops and unresolved-comment rechecks**: usually mechanical; only escalate
+  to a higher-smartness model if a review comment surfaces genuine ambiguity.
+- When in doubt, the executing agent should state which tier it picked and why, so it can
+  be verified rather than silently over- or under-spending.