Story validation can still block on GitHub device auth via `pdd fix <story>` and story prompt-link caching (detect_change called unwrapped)

[niti-go]

Problem

Issue #1923 fixed the non-interactive device-login hang for story validation by scoping PDD_NO_INTERACTIVE inside run_user_story_tests, which protects pdd detect --stories, pdd change, agentic change, and drift. However, two sibling story-validation paths call detect_change directly WITHOUT that guard and can still reach the interactive GitHub device-flow and block forever in a non-TTY agent/CI shell:

• run_user_story_fix (used by pdd fix <story>) calls detect_change directly before it later delegates to the protected runner.
• The story prompt-link caching path (used by story link/add flows) calls detect_change directly.

Why this is bad

These are the same failure mode as #1923 in adjacent commands; an agent running pdd fix <story> in a non-interactive shell with cloud credentials present but no cached JWT can still hang on a device code.

Evidence

Found during adversarial review of PR #1959. The guard lives in run_user_story_tests; the two direct detect_change call sites bypass it.

Expected behavior

Every story-validation path (including pdd fix <story> and story prompt-link caching) must be non-interactive-safe in a non-TTY shell — no device-login flow starts; missing credentials fail closed with actionable guidance.

Proposed solution

Relocate the non-interactive scoping down into detect_change itself (or a small shared context manager reused by all detect_change call sites in story validation), so the guard is truly shared rather than living only in run_user_story_tests. This would also cover standard pdd detect prompt-change mode. Red/green: pdd fix <story> in a non-TTY shell with cloud creds and no cached JWT must not start device auth and must exit non-zero with guidance.

References

• Triggering issue: pdd detect --stories blocks on interactive GitHub device auth in non-interactive runs #1923
• PR: fix: prevent story detection device auth in non-tty #1959