docs: document combo invocation by exact model name (#7992)

[maxmad64bis]

Summary

Persisted combos (Settings → Combos) are only used when the client sends their exact name in the model field — auto/auto/* is a separate zero-config router that does not consult persisted combos. The confusion produced two issues (#7992, #7111): reporters' clients kept sending model: "auto" while their combos were never reached, and openrouter/auto 402s were a real paid OpenRouter product, not an alias.

This PR documents the invocation contract on three surfaces (no runtime change):

• Combos page guide (Getting Started with Combos): new info block + 4 i18n keys (usageGuideInvoke*)
• docs/routing/AUTO-COMBO.md: new section Calling a Custom Combo From a Client
• docs/getting-started/AUTO-COMBO-GUIDE.md: FAQ Can I force a specific provider? now mentions the exact-name requirement
• changelog fragment (changelog.d/maintenance/10779-combo-invocation-docs.md)

Related Issues

• Closes fix(backend): combos not used — enabled providers not seen by the router #7992
• Related to fix(api): auto/* routing aliases fail from Hermes custom OpenAI endpoint #7111

Validation

Choose the change type and focused loop from the Contribution Golden Path. The full unit suite, Vitest, the 60% coverage gate, and the production build all run in CI on this PR (#8329):

• Change type: UI / i18n / docs
• Focused tests and category gates from the golden path: check:docs-sync, check:docs-counts, check:doc-links, check:fabricated-docs, i18n:check-ui-coverage, check:changelog-integrity — PASS locally
• npm run lint — PASS locally
• Reconciled with the current active release base (fork synced to upstream tip 3d7ed7aa8, branch rebased, focused checks rerun afterward)
• Production-code changes include a new or updated automated test in this PR: tests/unit/combo-guide-invocation-keys.test.ts (2 tests)
• SonarQube is temporarily opt-in while the private project has no quota; it is not a PR gate.

Known base-red: check:env-doc-sync fails on the base tip itself (verified on a clean base worktree). ⚠️ base-red inherited: #9985

Tests Added Or Updated

• tests/unit/combo-guide-invocation-keys.test.ts — new: asserts the 4 usageGuideInvoke* i18n keys exist in en.json with the expected copy, and that ComboUsageGuide (page.tsx) references each key via getI18nOrFallback. Run: node --import tsx/esm --test tests/unit/combo-guide-invocation-keys.test.ts → 2/2 PASS.

Coverage Notes

• The only production-code change is a static copy block in src/app/(dashboard)/dashboard/combos/page.tsx (4 getI18nOrFallback calls) + 4 new en.json keys (i18n files excluded from the test-policy gate by design). The new test covers the catalog keys and their page references; no covered branches are removed, so no coverage movement is expected.

Reviewer Notes

• No runtime behavior change — getComboForModel() resolution order is untouched.
• Local check:dashboard-typecheck was inconclusive in the dev environment (stale node_modules before the fork sync; process died after npm install); the gate runs authoritatively in CI.
• lint:md reports only pre-existing errors (CHANGELOG.md, AUTO-COMBO.md lines 7/10, AUTO-COMBO-GUIDE.md lines 7/50) — none on the added lines.
• The grand-public FAQ line and the docs section deliberately reuse the same wording as the UI note so all three surfaces stay consistent.
• UI render not screenshot-tested: the added block is static copy following the exact patterns of the existing guide blocks (same i18n helper, same design-system classes), covered by lint + the new unit test.