feat: x402 payment protocol support (#11)

* feat: add x402 wallet management and payment signing CLI

Adds mcpc x402 subcommands: init, import, list, remove, sign.
Wallets stored in ~/.mcpc/wallets.json. Signing parses PAYMENT-REQUIRED
headers and produces PAYMENT-SIGNATURE for x402 payment protocol.

* feat: add x402 commands to mcpc --help output

* feat: add x402 auto-payment middleware for direct and bridge connections

Implement automatic x402 payment signing when calling paid MCP tools.
The --x402 global flag enables auto-payment using the default wallet.

Architecture:
- Shared signer module (src/lib/x402/signer.ts) for EIP-3009 typed data signing
- Fetch middleware (src/lib/x402/fetch-middleware.ts) wraps SDK's fetch with:
  - Proactive signing: reads _meta.x402 from cached tools list, signs before request
  - 402 fallback: if server returns HTTP 402, parses PAYMENT-REQUIRED header, signs, retries once
- Bridge receives wallet credentials via IPC (set-x402-wallet message type)
- Bridge maintains its own tools cache for proactive signing (populated on connect,
  updated via listChanged notifications)
- Direct connections (withMcpClient) only support 402 fallback (no persistent tools cache)
- customFetch option threaded through transports.ts -> factory.ts -> SDK transport

Known issues and TODOs:
- --x402 is currently a boolean flag (always uses 'default' wallet). Supporting
  --x402 [wallet-name] with optional value is blocked by Commander.js not being
  able to disambiguate optional option values from subcommands. Options considered:
  (a) pre-process args to convert '--x402 name' to '--x402=name' before Commander,
  (b) use separate --x402-wallet <name> option, (c) strip --x402 from Commander
  args and re-inject via closure. All approaches were prototyped but add significant
  complexity. For now --x402 uses the default wallet; named wallets can wait.
- No unit tests yet for signer.ts and fetch-middleware.ts
- No E2E test with the demo x402 server
- Interactive shell does not support --x402 (only direct commands and sessions)
- Direct connections don't have proactive signing (no tools cache), only 402 fallback
- Pre-existing flaky test: sessions/bridge-resilience occasionally fails (unrelated)

* refactor: simplify x402 to single wallet

Drop multi-wallet support (named wallets, --name options, defaultWallet)
in favor of a single wallet stored in ~/.mcpc/wallets.json with a
versioned format { version: 1, wallet?: WalletData }.

- Remove wallet names from WalletData, X402WalletCredentials, SessionData
- Replace SessionData.walletName with x402 boolean flag
- Simplify wallets.ts CRUD to operate on single wallet
- Rename 'list' command to 'info', remove --name from all x402 commands
- Update bridge IPC: --wallet <name> arg becomes --x402 boolean flag
- Update all callsites (bridge-manager, sessions, helpers, parser, CLI)

This avoids Commander.js optional-arg parsing complexity for --x402
[wallet-name] and matches the actual use case (one wallet per user).
Multi-wallet can be added later with --x402-wallet <name> if needed.

* docs: add x402 agentic payments section to README

* docs: add Base link and warning emoji in x402 section

Author: MQ37

README.md

@@ -61,7 +61,8 @@
     "ora": "^9.0.0",
     "proper-lockfile": "^4.1.2",
     "undici": "^7.22.0",
-    "uuid": "^13.0.0"
+    "uuid": "^13.0.0",
+    "viem": "^2.46.3"
   },
   "devDependencies": {
     "@types/jest": "^30.0.0",