feat: add ~/.voipirc.json config file support

[dean-harel]

Closes #4

Implements persistent configuration via JSON files so users don't have to pass --provider, --voice, --lang, and --rate on every invocation.

Config file locations (precedence order)

1. $PWD/.voipirc.json — project-local override
2. $HOME/.voipirc.json — user global (primary)
3. $XDG_CONFIG_HOME/voipi/config.json — XDG compliant
4. $HOME/.config/voipi/config.json — XDG fallback

Schema

All fields optional. Unknown keys are ignored. Malformed JSON warns to stderr and falls back to the next path.

{
  "provider": "edge-tts",
  "voice": "en-US-GuyNeural",
  "lang": "en",
  "rate": 1.2
}

Precedence (highest to lowest)

• Method options (speak({ voice: "X" }))
• Constructor options (new VoiPi({ providers: [...] }))
• CLI flags (--voice, --provider, etc.)
• Config file (~/.voipirc.json)
• Hardcoded defaults (auto chain, provider defaults)

Changes

• src/_config.ts — config discovery and loading
• src/voipi.ts — merge config defaults into speak / save / synthesize; use config provider when no constructor options given
• src/cli/index.ts — add --show-config flag; mention config file in --help
• test/config.test.ts — vitest coverage for loading, precedence, malformed JSON fallback, and VoiPi integration
• test/voipi.test.ts — mock loadConfig to stay deterministic
• README.md & packages/pi/README.md — document config file usage

Notes

• Config is read once per VoiPi construction (not module scope), so Pi's /reload and long-running processes pick up changes without module cache issues.
• No config get/set/list subcommands — users edit the JSON file manually (per issue scope).
• No per-provider internals leaked in config.
• No hot-reload/watch — restart process to pick up changes.

[vercel]

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