Pin the provider/model to each session and restore it on switch

[plombeer31]

Problem

The active text provider and its chat model are a single global setting, so every session silently follows whatever was last picked. Run one project on openrouter/z-ai/glm-5.2 and another on local-llama, switch between their sessions — and both quietly use the same model. A session has no memory of what it was driven with.

Change

Each session now records the provider/model it runs on, and switching into a session re-applies it:

• Stamp: executeTurn writes metadata.llm = { providerId, chatModel } with the post-turn save, resolved from the live config at turn start (deliberately not the fallback chain's emergency substitute — the session should remember the operator's choice, not the outage). Every origin funnels through executeTurn, so TUI/scheduler/Telegram/HTTP turns all stamp.
• Stamp on pick: the TUI also stamps the open session the moment a model is chosen in the LLM panel (providers_select_chat_model / providers_set_active_text), so a choice made between turns survives switching away before the next message.
• Restore: switchSession compares the target session's stamp against the active config (planModelRestore, a pure function) and, when they differ, re-applies it by emitting the LLM panel's own bus actions — config persistence, provider reload and panel refresh all happen in the code path that already owns them. A transcript line says what was restored.
• Degradation: a stamped provider that has since been removed changes nothing and says so; sessions without a stamp (pre-existing rows) keep the current model; malformed metadata reads as "no stamp", never a crash.

Scope note: model resolution stays global at runtime — the restore happens when you enter a session. A scheduled/background turn on another session still uses the currently active model; per-turn model plumbing through the engine would be a much larger change and is intentionally out of scope here.

Tests

• readSessionLlmStamp: well-formed / model-less / malformed metadata
• planModelRestore: no stamp, already active, cross-provider select, legacy model field, model-less activate, deleted provider
• runtime integration: a turn stamps the stored session with the active provider