Fix macOS keychain credentials (#3) and ship missing rename.cjs (#2)

[mozlet]

Summary

Bundles two fixes that make the switcher work out of the box on macOS:

1. macOS login-keychain credential bridge (fixes #3)

On macOS, Claude Code stores OAuth credentials in the login keychain (generic
password, service Claude Code-credentials), not ~/.claude/.credentials.json.
lib/store/io.cjs now routes credential read/write through the security CLI,
but only on darwin, when the credentials file is absent, and a keychain
item exists — so Linux/WSL/Windows and file-based macOS installs are unaffected.
A pre-switch keychain backup is written so a bad write is recoverable.

This is the approach from @shackstack's #5, applied verbatim.

2. Ship lib/actions/rename.cjs (fixes #2)

cc-switch.cjs requires lib/actions/rename.cjs (added with the account-alias
feature), but the file is missing from the package.json files whitelist and
from install.cjs, so npm/npx installs crash with MODULE_NOT_FOUND. Added to
both. Overlaps with #4.

Testing (macOS, Darwin, Node 18+)

• cc-sync-oauth / cc-switch read the active account from the keychain (no more ENOENT)
• account list renders
• security add-generic-password -U write roundtrip verified
• npm pack now includes lib/actions/rename.cjs

Notes

• Bundles the two currently-stalled fixes (Include lib/actions/rename.cjs in published package (fixes #2) #4 packaging, Support macOS login keychain for credentials (fixes #3) #5 keychain) into one merge; happy to split if you prefer.
• Includes a version bump to 0.4.0 — drop it if you'd rather bump separately.

🤖 Generated with Claude Code

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[Leuconoe]

@mozlet I'm sorry. For some unknown reason, I didn't receive the PR notification. Since there have been changes, please resolve the conflicts and resubmit the PR. Thank you.