Dedup Claude /model list

[asujithan]

Problem

ucode claude → /model showed duplicate "Opus 4.8" rows (one from ANTHROPIC_MODEL, one from ANTHROPIC_DEFAULT_OPUS_MODEL) that both resolved to the same gateway-routable id. Visually noisy and confusing — users couldn't tell which row to pick.

Fix

• Stop setting ANTHROPIC_MODEL in render_overlay. The "Default (recommended)" row now resolves through the pinned ANTHROPIC_DEFAULT_OPUS_MODEL family alias instead of standing as its own duplicate row.
• Actively prune stale ucode-managed model env keys in write_tool_config. deep_merge_dict doesn't remove keys absent from the overlay, so without this any ANTHROPIC_MODEL (or short-lived _NAME companions) written by older ucode versions would linger in ~/.claude/settings.json forever and the dupe would never go away. Pruning is scoped to a fixed allowlist (CLAUDE_MANAGED_MODEL_ENV_KEYS) so user-owned env vars are untouched.

Test plan

[x] uv run pytest tests/test_agent_claude.py (53 passed)
[x] uv run pytest (762 passed; 2 pre-existing env-only failures in pi/opencode user-agent e2e tests unrelated to this change)
[x] uv run ruff check .
[x] Manual: ucode claude shows 4 rows in /model; "Default (recommended)" launches in opus; /model sonnet switches families; ucode claude -- --model haiku starts in haiku