feat: keep the chat tabs you left open, and keep them with you

Reopening the app closed every project tab: the copilot's tab bar lived in
RAM, so a reload dropped it and each conversation had to be found again from
its project board.

The set of open tabs is now a column on the session row, `chat_sessions.is_open`
(additive, `ensure_column`), restored by `GET /api/sessions/open` and written by
`PUT /api/sessions/{id}/open`. Not localStorage: that store is per-origin, so on
a shared laptop one member's tabs would greet the next, whereas the owner table
sits in their own encrypted file and follows them to another device. Which tab
is *selected* stays in sessionStorage — that one is genuinely per window, and a
shared value would have two windows fighting over it.

`is_open` defaults to 0 and `chat_sessions::create` never sets it: every `/new`
leaves its predecessor behind and every system-agent pass mints a row, so the
opposite default would restore a bar full of conversations nobody opened. Only
the copilot writes the column, at the moment it opens the tab. A reset moves the
flag rather than copying it — `POST /api/sessions` now returns the new id and
`new_session` carries it, and the old row is closed as the new one opens, or the
source would restore twice and a later close would clear the stale row.

Closing a tab clears the flag and nothing else: the conversation is kept and
comes back with its history when the project is reopened.

crates/skald-core/src/db/mod.rs

@@ -776,12 +776,24 @@ pub async fn create_owner_tables(pool: &SqlitePool) -> Result<()> {
             agent_id         TEXT    NOT NULL DEFAULT 'main',
             is_interactive   INTEGER NOT NULL DEFAULT 1,
             is_ephemeral     INTEGER NOT NULL DEFAULT 0,
+            is_open          INTEGER NOT NULL DEFAULT 0,
             run_context      TEXT,
             created_at       TEXT    NOT NULL DEFAULT (datetime('now'))
         )",
     )
     .execute(pool)
     .await?;
+    // Which conversations the copilot shows as tabs — persisted here rather than in
+    // the browser so the set follows the person (a shared laptop can't leak one
+    // member's tabs to another) and stays inside their encrypted file.
+    //
+    // The default is deliberately **0**, not 1: every `/new` leaves its previous
+    // session behind, and every system-agent pass creates one, so `DEFAULT 1` would
+    // turn every historical row on an existing box into a tab at the next login.
+    // For the same reason `chat_sessions::create` doesn't set it — it also serves
+    // cron, channels and system agents. Only the copilot writes this column, at the
+    // moment it opens the tab.
+    ensure_column(pool, "chat_sessions", "is_open", "INTEGER NOT NULL DEFAULT 0").await?;
 
     sqlx::query(
         "CREATE TABLE IF NOT EXISTS chat_sessions_stack (