chore: rename packages/ to crates/

Move all 29 workspace members from packages/<name>/ to crates/<name>/.
Updates: workspace Cargo.toml (members + path deps), justfile, root
CLAUDE.md, scripts/build/CARGO_INSTALL.md, docs/architecture/crates.md
(renamed from packages.md), structural references in docs/architecture
and docs/configuration, per-crate CLAUDE.md self-references. Historical
plans, reports, and building/ docs are left untouched.

No behavior change; just check-all stays green and fermata tests pass.

tests/fixtures/valid_basic.yaml

@@ -0,0 +1,37 @@
+version: "0.1"
+
+sessions:
+  - id: "session-1"
+    title: "Basic Test Session"
+    created_at: "2024-01-01T00:00:00Z"
+    participants:
+      - id: "user-1"
+        kind: user
+        display_name: "Test User"
+      - id: "assistant-1"
+        kind: assistant
+        display_name: "Test Assistant"
+    messages:
+      - id: "msg-1"
+        session_id: "session-1"
+        role: user
+        content: "Hello, assistant!"
+        created_at: "2024-01-01T00:00:01Z"
+      - id: "msg-2"
+        session_id: "session-1"
+        role: assistant
+        content: "Hello! How can I help you today?"
+        created_at: "2024-01-01T00:00:02Z"
+        parent_id: "msg-1"
+
+responders:
+  keyword_map:
+    hello: "Hi there!"
+    help: "I can assist you with various tasks."
+  default_strategy: echo
+
+streaming:
+  enabled: true
+  tokens_per_chunk: 5
+  chunk_interval_ms: 100
+  jitter_ms: 20