feat: add CEP-8 explicit payment gating to cvmi call

[abhayguptas]

Add CEP-8 Payment Lifecycle Support to cvmi call

Description

This PR brings full CEP-8 payment handling to the cvmi call command, bringing it up to parity with the contextvm-site client. Previously, calling a paid MCP endpoint via the CLI would fail or hang because the CLI had no awareness of notifications/payment_required.

This update upgrades @contextvm/sdk to 0.13.0 and utilizes its native withClientPayments middleware to handle all the heavy lifting (PMI capability advertisement, NIP-47 heartbeats, and waiting for server settlement).

Changes

• SDK Upgrade: Bumped @contextvm/sdk to 0.13.0
• Transport Wrapper: Wrapped NostrClientTransport in createRemoteClient with withClientPayments middleware.
• CLI Payment Handler: Added a new CliPaymentHandler (mirroring UiOnlyPaymentHandler in contextvm-site). Instead of auto-paying, this handler simply prints the parsed lightning invoice to the terminal using standard UI tokens (BOLD, CYAN, DIM), and waits for human payment out-of-band while the SDK background loop awaits payment_accepted.
• Payment Modes: Introduced the --payment-mode flag.
  • transparent (Default): Pauses execution, prints the invoice to the terminal, and waits for settlement.
  • explicit_gating: Designed for AI agents. If the server throws a -32042 Payment Required error, the CLI immediately outputs the raw payment schema as structured JSON to stdout and throws an exit code 2.

Usage Example

Human Usage (Transparent):

$ cvmi call <paid-server> <tool> --payment-mode transparent
# Output:
# ⚡ Payment Required
# ──────────────────────────────────────────────────
#   Amount:      50
#   Description: Paid tool execution
#   PMI:         bitcoin-lightning-bolt11
#   Expires in:  300s
# ──────────────────────────────────────────────────
#   Invoice:
#   lnbc...
# ──────────────────────────────────────────────────
#
# Pay the invoice above. The CLI is waiting for confirmation...

Agentic Usage (Explicit Gating):

$ cvmi call <paid-server> <tool> --payment-mode explicit_gating
# Output:
# { "instructions": "...", "payment_options": [ ... ] }
# Exit code: 2