MAI-48: nodemaiass - auto-pick AI commit model by diff size

[vsmash]

Summary

CLI half of MAI-48 (sibling PR in maiass-proxy for the worker half).

Replaces the single-source AI commit model picker (MAIASS_AI_MODEL || 'gpt-3.5-turbo') with a layered resolver in lib/commit.js:

1. Override — MAIASS_AI_COMMIT_MODEL (new env var) wins absolutely. Skips all picker logic.
2. Legacy back-compat — MAIASS_AI_MODEL (unchanged) used when commit override is unset. Preserves prior behaviour for users who configured it explicitly.
3. Auto-pick — when neither is set, ladder by post-truncation diff char count:
   • < 30000 chars -> gpt-4o-mini (tier 1, common case given the 8000-char default cap)
   • 30000..100000 chars -> gpt-4-turbo (tier 2)
   • > 100000 chars -> gpt-4o (tier 3)

Tiers 2 and 3 only fire when a user raises MAIASS_AI_MAX_CHARACTERS above 30000 — the ladder is intentionally forward-compatible with future cap increases.

Under MAIASS_DEBUG=true the picker logs the chosen tier, diff length, and threshold range.

Changes

• lib/commit.js — new exported pickCommitModel(diffLength, env) helper; wired into the commit suggestion flow after diff truncation; debug log added
• lib/maiass-variables.js — register MAIASS_AI_COMMIT_MODEL
• lib/config-manager.js + lib/config-command.js — add MAIASS_AI_COMMIT_MODEL to the AI category groupings
• test/unit/commit-model-picker.test.js — 13 cases covering override, legacy fallback, tier boundaries (0, 29999, 30000, 100000, 100001, 250000), defensive negative input

Out of scope

• Worker changes (sibling PR in maiass-proxy).
• MAIASS_AI_MAX_CHARACTERS itself — unchanged.
• Bash CLI parity — separate follow-up.
• lib/changelog-cleanup.js — per MAI-47 thread, cleanup chunks are too small to need the ladder; already has its own MAIASS_AI_CHANGELOG_MODEL.

Jira

https://velvary.atlassian.net/browse/MAI-48

Test plan

• npm run test:unit — 92 pass (was 79 before this branch; 13 new tests added)
• Manual smoke: MAIASS_AI_COMMIT_MODEL=gpt-4o MAIASS_DEBUG=true maiass -c — confirm debug log shows "override via MAIASS_AI_COMMIT_MODEL"
• Manual smoke: unset both env vars, run with small staged diff — confirm picker logs tier 1 / gpt-4o-mini
• Manual smoke: with MAIASS_AI_MODEL=gpt-3.5-turbo set and MAIASS_AI_COMMIT_MODEL unset, confirm picker logs "legacy MAIASS_AI_MODEL"