fix(distribution): make claimed installer reruns idempotent

[teckedd-code2save]

Failure found by clean-host acceptance

The first Distribution Acceptance run proved:

• checksum verification
• fresh install
• loopback-only binding
• one-time claim
• claim consumption
• private-mode post-install verification
• host execution / PTY / MCP / OAuth readiness

It then failed on the second installer run after claim.

Root cause: the healthy-existing-instance path had a duplicated sed JSON parser with an extra escaping layer. create-install-claim.cjs correctly returned stage: already_claimed, but the installer parsed an empty stage and returned "unexpected claim state".

Fix

• introduces one json_string_field helper
• routes all 8 claim JSON field reads through that helper
• removes the divergent fresh-vs-existing parser copies
• adds a regression assertion that the installer uses the shared parser
• refreshes the published installer SHA-256

After merge/deploy, the existing Distribution Acceptance workflow will run automatically again because scripts/install changed.