Normalize session_id into ws_id as sole persistent identity (#29)

* Normalize session_id into ws_id as sole persistent identity

Eliminate the separate session_id concept. The workstream ID (ws_id) is
now the single identity used for both real-time routing and conversation
persistence, removing a layer of indirection that was 1:1 in practice
and buggy on resume (stale pointers, orphaned rows).

Schema changes (migration 006):
- Drop sessions table; add alias/title columns to workstreams
- Rename conversations.session_id → ws_id
- Rename session_config table → workstream_config (ws_id column)
- Data migration remaps existing conversations to ws_id

Storage/API renames:
- register_session → register_workstream (already existed, merged)
- save_message/load_messages now keyed by ws_id
- resolve_session → resolve_workstream
- ChatSession.session_id property → ws_id
- ChatSession.resume_session() → resume()
- resume_session field → resume_ws
- SessionResumedEvent → WorkstreamResumedEvent
- /api/sessions → /api/workstreams/saved
- /sessions slash command → /workstreams
- --session-retention-days → --retention-days

Channel eviction recovery simplified: reuses old ws_id directly
instead of get_session_id_by_ws() reverse lookup.

* Fix Copilot review feedback: stale session wording in docs, regenerate OpenAPI spec

- docs/channels.md: "resumes the session" → "resumes the workstream",
  "Session resumed:" → "Resumed:", "old session was pruned" → "old
  workstream was pruned"
- docs/api-reference.md: "Each session object" → "Each saved workstream
  object", field descriptions updated, removed stale node_id field
- sdk/typescript/openapi-server.json: fully regenerated from Python
  models — removes all stale session_id properties from WorkstreamInfo,
  DashboardWorkstream, CreateWorkstreamResponse schemas

docs/api-reference.md

@@ -515,8 +515,8 @@ Returns a list of all active workstreams.
 ```json
 {
   "workstreams": [
-    {"id": "abc123", "name": "default", "state": "idle", "session_id": "a1b2c3d4e5f6"},
-    {"id": "def456", "name": "hacker-news", "state": "thinking", "session_id": "c5d6e7f8a9b0"}
+    {"id": "abc123", "name": "default", "state": "idle"},
+    {"id": "def456", "name": "hacker-news", "state": "thinking"}
   ]
 }
 ```
@@ -528,22 +528,21 @@ Each workstream object:
 | `id`         | string      | Unique workstream routing identifier                   |
 | `name`       | string      | Display name (alias if set, otherwise `ws-xxxx`)       |
 | `state`      | string      | Current state (see state values above)                 |
-| `session_id` | string/null | Session ID of the workstream's `ChatSession`, used for deduplication against `/v1/api/sessions` |
 
 ---
 
-### `GET /v1/api/sessions`
+### `GET /v1/api/workstreams/saved`
 
-Returns a list of saved sessions from the database, ordered by most recently
+Returns a list of saved workstreams from the database, ordered by most recently
 updated.
 
 **Response:**
 
 ```json
 {
-  "sessions": [
+  "workstreams": [
     {
-      "session_id": "a1b2c3d4e5f6",
+      "ws_id": "a1b2c3d4e5f6",
       "alias": "refactor",
       "title": "JWT Authentication Refactor",
       "created": "2026-03-01 10:00:00",
@@ -554,18 +553,16 @@ updated.
 }
 ```
 
-Each session object:
+Each saved workstream object:
 
 | Field           | Type        | Description                                |
 |-----------------|-------------|--------------------------------------------|
-| `session_id`    | string      | Unique 32-char hex UUID session identifier |
+| `ws_id`         | string      | Unique workstream identifier               |
 | `alias`         | string/null | User-assigned short name                   |
 | `title`         | string/null | LLM-generated title                        |
-| `created`       | string      | ISO timestamp of session creation          |
+| `created`       | string      | ISO timestamp of workstream creation       |
 | `updated`       | string      | ISO timestamp of last message              |
-| `message_count` | int         | Number of messages in the session          |
-| `node_id`       | string/null | Server node that created the session       |
-| `ws_id`         | string/null | Workstream the session belongs to          |
+| `message_count` | int         | Number of messages in the workstream       |
 
 ---
 
@@ -721,7 +718,7 @@ All fields are optional. The body can be empty or an empty JSON object.
 | `name`           | string | auto    | Workstream display name                                        |
 | `model`          | string | default | Model alias from the registry (`[models.*]`)                   |
 | `auto_approve`   | bool   | false   | Auto-approve all tool calls for this workstream                |
-| `resume_session` | string | ""      | Session ID to resume atomically during creation (empty = fresh)|
+| `resume_ws`      | string | ""      | Workstream ID to resume atomically during creation (empty = fresh)|
 
 **Response (success):**