Follow-on stacks

A stack marked follow_on: true is parked work: PRs to get to later, not part of the stacks going up now. They’re reviewed like any other stack but held back from publishing, always drawn in yellow, kept at the end of the repo, and left out of every total.

stacks:
  - title: The feature itself
    starts_at: "Add the feature flag"
  - title: Later
    starts_at: "Polish the feature"
    follow_on: true             # parked: held back from the stacks going up now

A follow-on stack: below the dashed divider, faded and hatched, and left out of the repo's totals.

What it does, and doesn’t

• It’s purely advisory. The PRs keep their numbers and position, and review, comments, Viewed and Draft/Ready all work as usual.
• There’s no promote action. When it’s time for the stack to go up, delete the follow_on: line.
• local-review plan leaves them out unless asked (--include-follow-on, or --stack <n> naming one). See The plan command. Agents don’t publish them unless the reviewer asks.
• Values. follow_on: true or yes. false, no or absent: a regular stack. Any other value is ignored, with a warning.
• Put them last. Follow-on stacks belong at the end of a repo’s stacks:. A regular stack after one gets a warning, since its PRs would sit on held-back work.
• Always yellow. A follow-on stack is yellow whatever its color: says, and yellow is reserved for them (Colours).
• Left out of totals, and mentioned beside them: +1 follow-on stack (2 PRs). The API’s GET /api/projects gives commitCount without them, and followOnCount. See Totals.

How the UI shows them

They use GitHub’s attention colours: #9a6700 text and #bf8700 lines in light mode, #d29922 in dark.

• Sidebar: a dashed yellow FOLLOW-ON · HELD BACK divider above the first one, then their boxes with a yellow border, faint yellow hatching and slightly faded rows (the text at 72%; full strength on hover and for the open PR; readiness badges and branch warnings never fade), and a dashed yellow follow-on badge on the stack title (not on each row, where it would squeeze the branch name).
• The repo heading’s viewed count leaves them out and shows their PRs beside it in a dashed yellow pill (0/11 (+2); the tooltip says how many of them are viewed). The open-threads counter still counts every PR.
• The commit page’s stepper shows the badge, with its pills in yellow.
• The project homepage: a dashed yellow left border, and the divider.
• The stack picture: a yellow border and label with a yellow follow-on tag. Its border and rows are drawn at 72% opacity (the label, the tag and a highlighted PR at full strength).