docs(provider-adapter): add architecture plan for multi-provider agent support

docs/provider-adapter/progress.md

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+# Agent Provider Adapter — Progress
+
+## Session Log
+
+### 2026-03-11 — Planning Phase
+
+**Duration:** ~30 min
+
+**What happened:**
+1. Explored 13+ files across Rust backend, TypeScript bridges, Svelte UI, and JS sidecar to map Claude-specific coupling
+2. Classified coupling into 4 severity levels (CRITICAL/HIGH/MEDIUM/LOW)
+3. Ran /ultra-think for deep architectural analysis — evaluated 3 design options for sidecar routing, message adapters, and settings UI
+4. Made 6 architecture decisions (PA-1 through PA-6)
+5. Created 3-phase implementation plan (16 + 5 + 3 tasks)
+6. Created planning files: task_plan.md, findings.md, progress.md
+
+**Architecture decisions made:**
+- PA-1: Per-provider sidecar binaries (not single multi-SDK bundle)
+- PA-2: Generic provider_config blob in AgentQueryOptions
+- PA-3: Per-provider message adapter files → common AgentMessage type
+- PA-4: Provider selection per-project with global default
+- PA-5: Capability flags drive UI rendering (not provider ID checks)
+- PA-6: Providers section in SettingsTab scroll (not inner tabs)
+
+**Status:** Planning complete. Ready for Phase 1 implementation.