Add multi-agent tabs, context snapshots, and tab UX

[DevMando]

Multiple concurrent agents, one per tab — each with its own conversation, project folder, model, and settings. The on-disk config becomes the defaults a new agent starts on; "Make Default for New Agents" is the only path that writes it.

• Per-agent object graph (AgentSession) mixing app-wide singletons with fresh per-tab instances; SessionManager owns the set. The three approval services are per-agent for correctness: the "don't ask again" bypass set, the SemaphoreSlim(1,1) prompt gate, and the single-assignment approval delegates each corrupt other tabs when shared.
• ConfigCoordinator is the sole caller of MandoCodeConfig.Save(), enforced by the MANDO001 build guard; config clones are a JSON round-trip followed by a mandatory ValidateAndClamp() (rebuilds McpServers with the case-insensitive comparer System.Text.Json drops).
• McpCoordinator owns the single McpClientManager on a host config with MCP always enabled; the per-agent flag controls tool attachment; /mcp-reload fans out to every agent's kernel.
• Context snapshots: capture the conversation the instant before a model switch clears it (and on demand via a tab's options menu), managed from a global left-rail panel; Import arms the active agent's next message. HistorySummarizer ports the harness's private compaction summary via the public GetHistoryAsync(); full history is stored for a future LLM-enhanced recap.
• Tabs named Agent 1/2/...; per-tab options menu (rename / snapshot / export / close, close greyed on the last agent); header model quick-switch dropdown; session status and events render as themed status chips (green = healthy, grey = event, gold = soft warning).

Fixes over the single-agent baseline: cross-agent approval isolation, WebView2 disposal on tab/window close, NRE on tab open/close (symmetric event handling), and accessibility names on icon-only buttons. Docs: CHANGELOG [Unreleased] and README updated.