docs: plan implementation agent Terra model

docs/superpowers/plans/2026-08-16-implementation-agent-terra-model.md

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+# Implementation Agent Terra Model Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Configure the project-local `implementation-agent` to use `gpt-5.6-terra` while preserving its existing reasoning effort.
+
+**Architecture:** Add one explicit model override to the existing Implementation Agent TOML profile. Do not introduce shared model policy, modify other profiles, or change the agent's instructions.
+
+**Tech Stack:** TOML, Python 3 `tomllib`, Git
+
+## Global Constraints
+
+- Set `model` to exactly `gpt-5.6-terra` only in `.codex/agents/implementation-agent.toml`.
+- Preserve `model_reasoning_effort = "extra high"`.
+- Do not modify other custom agents, user-global Codex configuration, solver production code, or agent instructions.
+- Do not add or modify contract tests or other test files.
+- Verify the change only through TOML parsing, exact-value assertions, and Git diff inspection.
+
+---
+
+### Task 1: Add the Implementation Agent model override
+
+**Files:**
+- Modify: `.codex/agents/implementation-agent.toml`
+- Test: none, per the approved design
+
+**Interfaces:**
+- Consumes: the existing project-local `implementation-agent` TOML profile
+- Produces: `model = "gpt-5.6-terra"` with the existing `model_reasoning_effort = "extra high"`
+
+- [ ] **Step 1: Add the model setting**
+
+Insert the model key between `sandbox_mode` and `model_reasoning_effort` so the profile header is:
+
+```toml
+name = "implementation-agent"
+description = "Implements FESA solver features in C++17/MSVC by following approved TDD-first implementation plans."
+sandbox_mode = "workspace-write"
+model = "gpt-5.6-terra"
+model_reasoning_effort = "extra high"
+```
+
+- [ ] **Step 2: Parse the profile and verify the exact values**
+
+Run:
+
+```powershell
+python -c "import pathlib, tomllib; p = tomllib.loads(pathlib.Path('.codex/agents/implementation-agent.toml').read_text(encoding='utf-8')); assert p['model'] == 'gpt-5.6-terra'; assert p['model_reasoning_effort'] == 'extra high'"
+```
+
+Expected: exit code `0` with no output.
+
+- [ ] **Step 3: Verify the change is surgical**
+
+Run:
+
+```powershell
+git diff --check
+git diff -- .codex/agents/implementation-agent.toml
+```
+
+Expected: no whitespace errors, and the profile diff contains only the added
+`model = "gpt-5.6-terra"` line.
+
+- [ ] **Step 4: Commit the configuration change**
+
+```powershell
+git add -- .codex/agents/implementation-agent.toml
+git commit -m "chore: use Terra for implementation agent"
+```