docs: PHASE_A_IMPLEMENTATION.md kickoff (provider infrastructure, ultraworkers#245/ultraworkers#246/ultraworkers#285)

Author: code-yeongyu

PHASE_A_IMPLEMENTATION.md

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+# Phase A: Provider Infrastructure (Implementation Kickoff)
+
+**Scope:** Formalize multi-provider routing and declarative config architecture. Critical path for Phases B-F.
+
+**Pinpoints in scope:** #245, #246, #285
+**Blocked by:** Phase 0 merge (GitHub OAuth, cargo fmt, clawcode-human approval)
+**Estimated effort:** 2-3 cycles
+**Target:** Merge-ready immediately post-Phase 0
+
+## #245 — Providers are hard-coded enum; no backend-swap capability
+
+**Acceptance Criteria:**
+- [ ] Providers defined as trait (not enum)
+- [ ] Factory/registry pattern allows runtime provider selection
+- [ ] Existing providers (Anthropic, OpenAI) are re-implemented as trait impls
+- [ ] Tests pass for all existing behavior
+- [ ] Zero breaking changes to public API
+
+**Implementation sequence:**
+1. Define `Provider` trait with core methods (chat completion, streaming, model listing)
+2. Implement trait for existing providers
+3. Add provider registry/factory
+4. Update CLI to accept `--provider` flag
+5. Regression tests
+
+## #246 — Provider selection logic is CLI-parsing only; no config source integration
+
+**Acceptance Criteria:**
+- [ ] Provider selection checks: 1) CLI flag, 2) env var, 3) settings.json, 4) default
+- [ ] settings.json schema includes `provider` field with subconfig
+- [ ] Env vars like `OPENAI_API_KEY` trigger automatic provider selection
+- [ ] Conflict resolution documented (CLI > env > config file > default)
+- [ ] Config merging tested
+
+**Implementation sequence:**
+1. Extend settings.json schema (add provider field, subconfig structure)
+2. Implement config-merge logic (priority order)
+3. Update `claw doctor` to validate provider config (#293 prerequisite)
+4. Integration tests
+
+## #285 — No declarative provider fallback; can't swap backends mid-session
+
+**Acceptance Criteria:**
+- [ ] `settings.json` supports `providers: [primary, secondary, fallback]` array
+- [ ] Streaming failures trigger automatic fallback to next provider
+- [ ] Session state is preserved across provider swap
+- [ ] User is notified of fallback event
+- [ ] `claw doctor --providers` shows fallback chain health
+
+**Implementation sequence:**
+1. Extend settings.json schema (providers array)
+2. Implement fallback logic in streaming handler
+3. Add state-preservation during swap
+4. User notification (log + maybe `--verbose` output)
+5. Integration tests with dual-provider setup
+
+## Dependency Graph
+
+```
+Phase 0 merge ──→ #245 (trait + registry) ──→ #246 (config integration) ──→ #285 (fallback)
+                           │                        │
+                           └────────────────────────┘
+                                  (parallel possible)
+```
+
+## Success Criteria (Phase A complete)
+
+- [ ] All three pinpoints (#245, #246, #285) have passing tests
+- [ ] `claw --provider openai` works
+- [ ] `claw --provider openai --fallback anthropic` works
+- [ ] settings.json with `{ "provider": "openai", ... }` is read correctly
+- [ ] `claw doctor --providers` validates all configured backends
+- [ ] Zero regression on existing Anthropic-only workflows
+- [ ] PR merges with zero cargo fmt warnings
+- [ ] clawcode-human approval granted
+
+## Next: Phase B (transport-layer + resilience)
+
+Once Phase A merges, Phase B begins with auto-compaction (#287, #288, #289) and streaming resilience (#223, #225, #229, #230, #232, #283, #287, #288, #289, #290, #291, #292).