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>

README.md

@@ -304,8 +304,8 @@ mishandled by real MCP clients:
 Plain streamable-HTTP — connect with the **default (legacy)** protocol era.
 Every tool here is still **runnable**: the override replaces only the
 *advertised* schema, and the flagship bare-`true` rides `get_temp` (which
-returns structured content) rather than `echo` — see the caveat under
-[Try it](#unportable-tool-schemas) below.
+returns structured content) rather than `echo` — see the ⚠️ caveat at the end
+of this section for why that placement matters.
 
 The Inspector is where a server author looks first, so a construct that will
 fail downstream is named here rather than passed through silently. All three