Support dynamic config loading or a CLI command to set user config

[mayashavin]

Description

Currently the config is loaded from a static polira.config.ts file in the project root. Users should be able to configure Polira more flexibly — either by loading config dynamically from multiple sources or via a dedicated CLI command to initialize/update their config.

Config Precedence (highest to lowest)

1. CLI flags (e.g. --mode, --size, --save) — per-invocation overrides, already partially exists
2. Environment variables (POLIRA_* prefix) — useful for CI/CD and Docker
3. Config file (polira.config.ts) — project-level settings, already works
4. Defaults (DEFAULT_CONFIG + zod .default() values) — zero-config baseline

Environment Variable Mapping

Flat POLIRA_* naming convention maps to nested config paths:

POLIRA_AI_TEXT_MODEL=gpt-4o-mini      → ai.textModel
POLIRA_AI_IMAGE_MODEL=dall-e-3        → ai.imageModel
POLIRA_IMAGE_SIZE=1024x1024           → image.size
POLIRA_IMAGE_STYLE="watercolor"       → image.style
POLIRA_STORAGE_PROVIDER=cloudinary    → storage.provider
POLIRA_WRITING_MODE=strong            → writing.defaultMode
POLIRA_MARKDOWN_COVER_FIELD=heroImage → markdown.coverField

Note: credential env vars (OPENAI_API_KEY, CLOUDINARY_*) stay as-is — they're secrets, not config. POLIRA_* vars are for behavioral settings only.

CLI Flags

Already partially exists (--mode, --size, --save, --upload). These overrides are applied per-command in each action handler. The gap is that they're applied inconsistently and not all config fields have corresponding flags.

Tasks

• Add POLIRA_* environment variable support to loadConfig() with deep-merge over config file
• Standardize CLI flag overrides across all commands (consistent pattern for applying flag → config)
• Add a polira config init command to interactively scaffold a config file
• Add a polira config set <key> <value> command to update individual config values
• Add a polira config show command to display the fully resolved config (all layers merged)
• Support global config at ~/.config/polira/config.ts as fallback (before defaults, after project config)
• Add config file auto-discovery (walk up directory tree to find nearest polira.config.ts)

Files

• src/core/config.ts — config loading logic, env var parsing, multi-source merge
• src/cli/commands/config.ts — new CLI command (init, set, show)
• src/cli/index.ts — register new command
• src/cli/commands/*.ts — standardize flag override pattern