feat: Add Client Authentication and Provisioning Plugin

[sortedcord]

This Pull Request implements the Client Authentication and Provisioning flow. It enables secure onboarding of new client devices and authorization by administrators using SSH Ed25519 key verification and age encryption.

Closes #20

Changes Proposed

1. Cryptographic Client Plugin (plugins/auth.sh)

• Implemented plugins/auth.sh handling both requester (me) and approver (trust) roles.
• Requester Flow (b me):
  • Generates SSH Ed25519 keys securely without passphrase under ~/.config/bootstrap-client/.
  • Submits hardware metrics (hostname, os) alongside the public key to /api/register.
  • Obtains user_code and challenge_nonce.
  • Polls /api/challenge/poll by signing the nonce using ssh-keygen -Y sign (namespace: bootstrap).
  • Decrypts the payload dynamically using age -d -i ~/.config/bootstrap-client/id_ed25519 and writes it to secrets.decrypted.
• Approver Flow (b trust <user_code>):
  • Resolves pending public keys via /api/pending/<user_code>.
  • Prompts the administrator for verification.
  • Digitally signs the public key using the admin's private key (ssh-keygen -Y sign) and submits the base64 signature alongside the fingerprint to /api/approve.

2. Router Integration (lib/routes.sh)

• Intercepts me and trust commands to lazy-load the auth plugin via run_plugin "auth".

3. Autocompletion (b.sh)

• Updated autocomplete options to support completing the me and trust commands after b.

4. Local Installation Updates (bootstrap.sh)

• Updated the local installation routine to copy the plugins/ directory (similar to installers/) to $BOOTSTRAP_DIR during local development bootstrapping.

5. Plugin Manifest (plugins.json)

• Registered auth in the local plugin catalog.

Verification & Testing

1. Run bootstrap.sh to sync the changes locally.
2. Initiated the requester flow:

   $ b me
   [INFO] Running plugin 'auth'...
   [INFO] Registering device with https://b.adityagupta.dev/auth...
   --------------------------------------------------------
   [SUCCESS] Device registration initiated successfully!
   Please authorize this device on your administrator machine using:
     b trust Oi1r2P --server https://b.adityagupta.dev/auth
   --------------------------------------------------------
   Verification Code: Oi1r2P
   --------------------------------------------------------
   [INFO] Waiting for administrator approval (polling every 5s)...

3. Verified that all generated signature files are securely removed from /tmp upon script termination.