rust: nexus-core storage, vault, keychain, export/import, data-dir migration

rust/core/migrations/002_github_protocol.sql

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+-- Migration 002: Add GitHub protocol support
+-- Adds the 'github' value to the protocol CHECK constraint and a github_config column.
+
+-- SQLite does not support ALTER TABLE to modify CHECK constraints or add columns
+-- with complex defaults atomically. We recreate the profiles table.
+
+-- Step 1: Create the new table with the updated constraint.
+CREATE TABLE IF NOT EXISTS profiles_new (
+    id TEXT PRIMARY KEY,
+    display_name TEXT NOT NULL,
+    description TEXT DEFAULT '',
+    protocol TEXT NOT NULL CHECK(protocol IN ('ssh', 'telnet', 'raw', 'github')),
+    host TEXT NOT NULL,
+    port INTEGER NOT NULL,
+    username TEXT DEFAULT '',
+    tags TEXT DEFAULT '[]',
+    folder TEXT DEFAULT '',
+    environment TEXT DEFAULT '',
+    notes TEXT DEFAULT '',
+    favourite INTEGER DEFAULT 0,
+    last_connected TEXT,
+    connect_timeout INTEGER DEFAULT 30,
+    read_timeout INTEGER DEFAULT 0,
+    terminal_font_family TEXT DEFAULT '',
+    terminal_font_size INTEGER DEFAULT 14,
+    terminal_scrollback INTEGER DEFAULT 10000,
+    terminal_cursor_style TEXT DEFAULT 'block',
+    ssh_config TEXT,
+    telnet_config TEXT,
+    raw_config TEXT,
+    github_config TEXT,
+    created_at TEXT NOT NULL,
+    updated_at TEXT NOT NULL
+);
+
+-- Step 2: Copy existing data (github_config will be NULL for legacy rows).
+INSERT OR IGNORE INTO profiles_new (
+    id, display_name, description, protocol, host, port, username,
+    tags, folder, environment, notes, favourite, last_connected,
+    connect_timeout, read_timeout,
+    terminal_font_family, terminal_font_size, terminal_scrollback, terminal_cursor_style,
+    ssh_config, telnet_config, raw_config, github_config,
+    created_at, updated_at
+)
+SELECT
+    id, display_name, description, protocol, host, port, username,
+    tags, folder, environment, notes, favourite, last_connected,
+    connect_timeout, read_timeout,
+    terminal_font_family, terminal_font_size, terminal_scrollback, terminal_cursor_style,
+    ssh_config, telnet_config, raw_config, NULL,
+    created_at, updated_at
+FROM profiles;
+
+-- Step 3: Drop old table and rename.
+DROP TABLE profiles;
+ALTER TABLE profiles_new RENAME TO profiles;
+
+-- Step 4: Record the migration version.
+INSERT INTO schema_version (version, applied_at) VALUES (2, datetime('now'));