Store API tokens in the OS keyring instead of plaintext credentials.yml

Summary

Today the CLI stores DNSimple API tokens in plaintext at ~/Library/Application Support/dnsimple/credentials.yml (and equivalents on Linux/Windows). Anyone with read access to that file — including malware running as the user, accidental backups, screen-shared terminals running cat, etc. — gets the token.

We should move tokens out of the file and into the OS keyring (macOS Keychain, Linux Secret Service, Windows Credential Manager), with the on-disk file storing only non-sensitive metadata (host, account ID, context name, user email).

Motivation

• Tokens are bearer credentials with full account access. Plaintext-on-disk is the lowest possible bar.
• Users on shared workstations, devs running with set -x, and folks who commit dotfiles have all hit this category of leak in other CLIs.
• This is the standard pattern for CLIs that handle long-lived API tokens (gh, aws, op, glab, doctl).

Reference: how gh does it

The gh CLI uses the same pattern we'd want:

• ~/.config/gh/hosts.yml stores hostname → user → non-sensitive metadata.
• The actual OAuth tokens live in the OS keyring, keyed by (hostname, user).
• A plaintext fallback exists for environments without a keyring (containers, headless CI), gated behind a configuration option.
• Resolution chain at runtime: env var → keyring(host, active user) → keyring(host) (legacy fallback).

The relevant code is in cli/cli internal/keyring and internal/config (AuthConfig.ActiveToken, TokenFromKeyringForUser). The underlying library is zalando/go-keyring (or 99designs/keyring, depending on the abstraction we want).

Proposed change

1. Add a keyring package wrapping a vendor library (likely zalando/go-keyring for its smaller surface area; revisit if we hit platform gaps).
2. Update the credentials file schema (post-#28) to store everything except the token. The keyring entry is keyed by (host, account_id) or (context_name) — to be decided in implementation.
3. Migrate existing plaintext tokens into the keyring on first run, then strip them from the file. Migration should be silent on success and leave a .bak file for one cycle.
4. Add a fallback mode for environments where the keyring is unavailable (CI, containers without dbus, SSH sessions without an unlocked keychain). The fallback should be opt-in, not automatic, and emit a warning the first time it's used. Suggested gate: DNSIMPLE_KEYRING=plaintext env var.
5. Honour DNSIMPLE_TOKEN env var override exactly as today — env always wins over keyring.

Open questions

• Library choice. zalando/go-keyring is simpler; 99designs/keyring (which gh uses) supports more backends including encrypted file fallback. Decision deferred to implementation.
• Keyring entry key. (host, account_id) is the most explicit but couples the keyring schema to the credentials schema. context_name is cleaner but means renaming a context requires re-keying the keyring. Decision deferred.
• Headless / CI behaviour. Hard-fail or fall back to plaintext with a warning? Recommend the env-var-gated fallback in step 4.
• Migration of existing plaintext tokens. Run on first load after upgrade, or on first explicit auth login? Recommend first load, with the .bak safety net.

Dependencies

• Blocked on dnsimple/dnsimple-cli#28. The named-context refactor is the right time to redesign the storage layer. Doing this before #28 lands would mean two disruptive credentials migrations back-to-back.

References

• dnsimple/dnsimple-cli#28 — named authentication contexts (blocker)
• dnsimple/dnsimple-cli#25 — the original auth fix that surfaced the credentials storage discussion

Start by reading the current credentials.yml handling and the named-context refactor in issue #28, which blocks this work. Trace how DNSIMPLE_TOKEN and stored credentials are resolved today. Done means tokens are handled through the agreed keyring design, migration and fallback behavior are specified, and tests cover the security-sensitive paths.