0.4-alpha: phases

Phases are the release, so AGENTS.md stops burying them. The six sections
that were `###`s inside "Task file format" — where they had split that
section's own prose in half, header table above and Assignee below —
become a `## Phases` section of their own, between "Pull requests" and
"Stages" because that is the order they are learned in. Nothing in them
changed: the diff is a move plus the section's opening paragraph.

Which gives the site three pages to cut rather than a tail nobody would
find at the bottom of "Task files": /concepts/phases/ (the model),
/concepts/running-a-phase/ (the branch, the beat, the halt, the ending)
and /concepts/phases-on-the-board/ (what the Board stops drawing, and the
Phases view that draws it instead). They sit after "PRs and review" in
the Concepts flow, which is the order AGENTS.md now reads in — the
manifest and the source cannot disagree about that without the build
saying so.

The landing page's third reason says it too, since "parallel work, zero
collisions" was only half of what the board now does, and the README's
opening paragraph gains the sentence it was missing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

manager/core/VERSION

@@ -1 +1 @@
-0.3-alpha
+0.4-alpha