feat(config): add config-reload to clear all session overrides

[tusharmath]

Summary

Replace the narrow model-reset command with a broader config-reload command that clears all session-scoped overrides — model, provider, and reasoning effort — restoring the global configuration in one step.

Context

The existing model-reset (mr) command only cleared _FORGE_SESSION_MODEL and _FORGE_SESSION_PROVIDER. Once reasoning effort was added as a third session override, users had no single command to return to a clean, config-driven state. The new config-reload (cr) command fills that gap, and the old aliases (model-reset / mr) are kept for backwards compatibility.

Changes

• Renamed model-reset built-in command to config-reload with an updated description reflecting its expanded scope
• Renamed _forge_action_model_reset → _forge_action_config_reload in the shell plugin and extended it to also clear _FORGE_SESSION_REASONING_EFFORT
• Declared _FORGE_SESSION_REASONING_EFFORT as a typed shell variable in config.zsh
• Updated the dispatcher to route config-reload|cr|model-reset|mr to the new action (backwards-compatible aliases preserved)
• Improved the informational messages to accurately describe what was cleared

Key Implementation Details

The dispatcher now matches four aliases (config-reload, cr, model-reset, mr) to the same handler, so existing shell history and muscle memory continue to work. The clear-all logic in _forge_action_config_reload checks all three session variables before deciding whether anything needs resetting, providing a more accurate "nothing to do" message.

Testing

# Start a shell session with the plugin loaded
source shell-plugin/forge.plugin.zsh

# Set session overrides
:model gpt-4o
:reasoning-effort high

# Verify overrides are active
echo $_FORGE_SESSION_MODEL          # gpt-4o
echo $_FORGE_SESSION_REASONING_EFFORT  # high

# Reset everything with the new command
:config-reload
# Expected: "Session overrides cleared — using global config"

# Verify all overrides are cleared
echo $_FORGE_SESSION_MODEL          # (empty)
echo $_FORGE_SESSION_PROVIDER       # (empty)
echo $_FORGE_SESSION_REASONING_EFFORT  # (empty)

# Verify backwards-compatible alias still works
:model gpt-4o
:model-reset
# Expected: same success message