feat: add --config to reconfigure credentials without running the agent

[Miller (bath-tub)]

Description

Adds openwiki --config, a settings-only entry point for changing the provider, API key, model, or LangSmith key.

Today, once ~/.openwiki/.env holds a complete set of credentials there is no way to change them from the CLI:

• --init maps to a documentation run (shouldStart: true), so it invokes the agent on mount. A user whose key is dead (expired trial, exhausted quota, wrong tier) just gets an error on boot with no path to switch keys.
• needsCredentialSetup() is only true when a value is missing, so InitSetup is never reached when the file is complete.
• Interactive chat has no key command — /provider only writes the provider + default model and prints "ensure KEY is set".

This fixes #23

What changed

• --config flag (commands.ts): new { kind: "config" } command. It errors if combined with --init/--update/--print/a message, and requires a TTY.
• reconfigure mode in InitSetup (credentials.tsx): walks every step (provider → key → model → LangSmith) even when values already exist.
• Non-destructive by design: on the API key and LangSmith prompts, an empty entry means keep the current value — completeSetup only writes non-null fields, so untouched secrets are never cleared. A saved custom model that isn't one of the provider presets is surfaced as a selectable "current" option so pressing Enter keeps it instead of silently falling back to the first preset.
• Never runs the agent (cli.tsx): the config flow renders the outcome and exits; the agent-run effect is gated on command.kind === "run". Env is now loaded for --config too so existing values are detected.
• Docs + help updated.

Testing

pnpm build, pnpm lint:check, pnpm format:check all pass. Drove the interactive flow through a PTY against an isolated HOME with a complete .env:

• Keep all (Enter through every step): every value preserved, no network call.
• Change key (type a new key, keep the rest): key replaced; provider, model, and LangSmith key preserved.
• Custom model + keep all: a non-preset OPENWIKI_MODEL_ID is preserved rather than overwritten.

Follow-ups (not in this PR)

• A /key (or /login) slash command for the in-session case.
• A hint on 429/insufficient_quota errors pointing at --config.
• Warn when the key being saved is shadowed by an exported shell var (loadOpenWikiEnv only fills process.env when unset, so an exported key wins over ~/.openwiki/.env on the next shell).

Made with Claude Code

[Colin Francis (colifran)]

Thanks for PR! This will be superseded by: #374 which makes init idempotent so re-running allows updating config.