1.9.0

Uniform milestone resolution with an override flag (milestone 50)

• Every skill that works inside an existing milestone now accepts an optional leading --milestone <milestone_id>, the exact directory name under milestones/, to run against that milestone instead of the current one. The override is honoured even when the pointer reads none, and a name with no requirements.md behind it stops with an error listing every milestone_* directory, never falling back to the pointer.
• The flag is stripped before a skill reads the rest of its arguments, so it never ends up in a Short Title, answer, goal, task name, or question.
• /specify-milestone-starting-state no longer takes a required milestone id and resolves like the other skills. /capture-milestone-principle-updates keeps its required positional id.
• /finish-current-milestone with an override finishes the named milestone and clears the pointer only when it names that milestone, leaving any other active milestone untouched.
• /complete-all-tasks passes the milestone directory it resolved to every complete-task agent, so an override reaches the agents too.
• Each affected skill's usage and its docs/skill-reference.md heading show [--milestone <milestone_id>], and the headless chains in docs/ways-of-using-cairn.md no longer pass a milestone id.

Full Changelog: 1.8.7...1.9.0