feat(auth): add support for multiple profiles per provider

[charles-cooper]

implements #5391

Summary

• Allow users to store multiple API keys for the same provider with named profiles (e.g., separate OpenRouter keys for different cost tracking)
• Add auth set-default command to swap a named profile to become the default

Changes

• Auth module: Added profile support to get/set/remove, plus profiles(), hasDefault(), setDefault() functions
• CLI commands: login prompts for profile name when default exists, logout/list show profile names, new set-default command
• Provider: parseModel() extracts profile from provider:profile/model format
• Config: Added optional profile field to provider schema
• Errors: Added ProfileNotFoundError and InvalidProfileNameError

Usage

# First login creates default profile
opencode auth login  # select openrouter, enter key

# Second login prompts for profile name
opencode auth login  # select openrouter, enter "work", enter key

# List shows profiles
opencode auth list
# OpenRouter (default) api
# OpenRouter:work api

# Use profile in model string
opencode --model openrouter:work/claude-sonnet-4

# Swap profiles
opencode auth set-default  # select openrouter, select "work"