Context

PR #1684 added enable_extended_context to CloudLlmConfig (TOML config). Per project Development Rules, new configuration options should also have a CLI argument for management.

Task

Add --extended-context CLI flag (boolean toggle) that sets llm.cloud.enable_extended_context = true at runtime without editing config.toml.

zeph --extended-context  # enables 1M context window for this session

Notes

• Should override the config file value (session-only)
• Display a cost warning similar to the wizard prompt
• Low priority: this is a cost-sensitive opt-in feature; config.toml is the primary interface

Follows up on #1649.