Summary

Currently there is no way to switch the active LLM provider at runtime via a chat command. The /model command allows switching models within the current provider, but changing the provider itself requires editing config.toml and restarting the agent.

Current Behavior

• /model [id|refresh] — switches model within the current provider
• /status — shows current provider and model
• Provider selection is static: set via [llm] provider = "..." in TOML at startup

Desired Behavior

A new /provider slash command that allows switching the active provider at runtime:

/provider              — list available configured providers
/provider <name>       — switch to a named provider (must be configured in TOML)
/provider status       — show current provider with stats (same as /status provider section)

Implementation Notes

• Add SlashCommand::Provider variant in crates/zeph-core/src/agent/slash_commands.rs (near /model at line 122)
• Handle in crates/zeph-core/src/agent/mod.rs alongside handle_model_switch (~line 2406)
• Only allow switching to providers that are already configured in the loaded config (no dynamic config mutation)
• After switching, reset model to the provider's configured default or require explicit /model <id> call
• Update /status output to reflect the new provider immediately
• TUI: emit a system status indicator during the switch (spinner + "Switching provider…" message, per TUI Rules)

Acceptance Criteria

• /provider lists configured providers with current one highlighted
• /provider <name> switches active provider; subsequent LLM calls use the new provider
• /provider <unknown> returns a clear error without crashing
• /status reflects the new provider after switch
• Works in CLI, TUI, and Telegram channels
• TUI shows a spinner during the switch operation