fix: rename the strict exit code, cover the config path, refresh the docs

Copilot review round 3.

- Rename the `--strict` failure from "invalid" to "unportable"
  (`EXIT_CODES.SCHEMA_UNPORTABLE`, envelope `schema_unportable`, and the
  message text). The module argues at length that these schemas ARE
  valid JSON Schema and are merely refused by some clients, so a
  machine-readable code of `schema_invalid` told automated callers the
  opposite. Done now, while nothing depends on it.

- Add a fixture-path integration case. Every other test built a
  `ServerConfig` by hand, so `resolveConfig`'s `rawToolSchemas`
  forwarding was uncovered and `unportable-schemas-http.json` was loaded
  by nothing — the shipped fixture could have lost its overrides with
  the suite still green. It now goes through loadConfig/resolveConfig
  and asserts the exact per-tool verdict the README claims.

- Refresh the CLI README's sample output, which still showed the
  pre-round-2 wording and fixture, and so dropped the contract warning
  that rewording existed to add. Captured from a real run rather than
  written by hand, plus a paragraph making the suggestion pattern
  explicit.

- Drop the self-referential "[Try it](#unportable-tool-schemas)" link in
  the root README, which pointed back at its own section.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UL9retfvAXRgvi6EWk4SY4
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/cli/src/error-handler.ts

@@ -9,6 +9,11 @@
  *  - 4: server unreachable (DNS, connect refused, timeout, fetch failure)
  *  - 5: tool error (`tools/call` returned `isError:true`, or tool not found)
  *  - 6: `--strict` found an error-severity tool-schema portability finding
+ *
+ * Note 6 is `SCHEMA_UNPORTABLE`, not "invalid": the whole premise of the lint
+ * is that these schemas ARE valid JSON Schema and are merely refused by some
+ * clients. Calling the outcome "invalid" would misreport it to the automated
+ * callers this map exists for.
  */
 export const EXIT_CODES = {
   OK: 0,
@@ -17,7 +22,7 @@ export const EXIT_CODES = {
   AUTH_REQUIRED: 3,
   UNREACHABLE: 4,
   TOOL_ERROR: 5,
-  SCHEMA_INVALID: 6,
+  SCHEMA_UNPORTABLE: 6,
 } as const;
 
 /** Machine-readable error envelope written as one JSON line on stderr. */