docs(design): rewrite flycheck modeline customization spec

Replaces the .ai/ draft (2025-11-14) with a corrected and tightened
version under docs/design/.  The earlier draft had stale line numbers
pointing at a modeline-config.el layout that no longer exists,
conflated Option 3's risky-local-variable requirement with Option 4's
inline (:eval ...) approach, and missed the active-window gating
convention used by the rest of the modeline.

The new spec uses concrete line refs against current code, calls out
flycheck-mode-line-color (which the old draft missed), recommends
calling flycheck-mode-line-status-text directly instead of returning
the nested (:eval ...) cons, and gates the segment to active window
for consistency with cj/modeline-vc-branch and cj/modeline-misc-info.

todo.org task points at the new path and drops the broken
docs/flycheck-modeline-customization-spec.org link.

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