docs(spec): add the modified-ticket indicator spec

Plan for surfacing unsaved local edits under the deferred save model. The cue is a background highlight (pearl-modified-highlight, inheriting diff-changed) over the field that changed (title, description, comment, or tag), plus the heading for fold-visibility, and a mode-line count of tickets with unsaved pushable work. It rides the existing dirty scan and viewer classification, so there's no new dirty logic, and it never mutates the buffer.

Two Codex review rounds and a follow-up design pass are folded in: the comment cue separates pushable own edits from non-pushable read-only ones, the update lifecycle and a single redecorate entry point are specified, and a cross-theme legibility survey picked diff-changed over diff-refine-changed.

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