feat(signer): MCP server with get_address, decode, sign, get_spend_limits

stdio transport only - no HTTP listener, no Provider, no outbound network.
Wires together decode.ts/policy.ts/sign.ts/audit.ts/keysource.ts (Tasks 2-6)
behind four tools: vector_signer_get_address, vector_signer_decode_transaction,
vector_signer_sign, vector_signer_get_spend_limits.

Two corrections needed beyond the brief's given code to reach its own stated
"pass 9, fail 0" outcome, both verified necessary by reproducing the failure
first:

- packages/signer/tsup.config.ts: target bumped es2020 -> es2022. index.ts's
  top-level `await server.connect(...)` is ES2022 syntax; esbuild hard-errors
  ("Top-level await is not available in the configured target environment")
  under the package's original es2020 target rather than attempting an unsafe
  downlevel transform. Scoped to the signer package only - the builder never
  uses top-level await and its own tsup.config.ts is untouched.
- tools.ts's decode_transaction response text: "(not signed)" and "would be
  signed" both contain the substring "signed", which collides with this same
  smoke test's own `!/signed/i.test(text)` check on every call, including the
  refused case. Reworded to "(read-only)" / "would be ALLOWED" - same meaning,
  no collision. Fixed the response text rather than loosening the test, which
  stays exactly as given.

Brief's own smoke test (9 tests) passes verbatim. Workspace-wide: typecheck 0,
test:unit 180/180 (no regressions), builder test:smoke still 26/23/0.

The audit-append-failure handling required by the task (a produced signature
must never be released if its audit record fails to write) is not yet in this
commit - see the follow-up commit.

package.json

@@ -13,6 +13,7 @@
     "test": "npm run test:unit",
     "test:unit": "node --import tsx --test packages/*/test/*.test.ts",
     "test:smoke": "npm run build && node --import tsx --test packages/builder/test/smoke/*.test.ts",
+    "test:smoke:signer": "npm run build && node --import tsx --test packages/signer/test/smoke/*.test.ts",
     "test:integration": "npm run build && node --import tsx --test packages/builder/test/integration/*.test.ts"
   },
   "devDependencies": {