#2638 gatorwalk-factory: integration harness robustness (key parsing, version-check failure, HOME writes)

Follow-up to #2619 (GW-7, PR 339), low findings from the verify-reviews code and adversarial reviews, all in gatorwalk-factory/integration/harness.ts and DESIGN.md:

1. newKey reads the generated key out of swamp's log text with a regex. A change to swamp's log line format fails every test at setup (loudly, so nothing is hidden). Prefer a structured source if swamp offers one, e.g. the method's logged properties in JSON output.
2. The swamp --version check is a shared promise; if the first call fails, every later test re-throws that same error, naming the first test's temp directory. Reset the cached promise on failure or fail each test with its own context.
3. DESIGN.md says the suite uses the caller's real HOME (config, login, deno npm cache) but not whether any command the suite runs writes there. Check and document it.

All low severity.