beads: extract the projection out of web/

web/beads.go held the one reading of the beads schema — the table
fingerprint, the lane bucketing, the ready rule, the status categories,
the transitive dependency walk, the event humanizer, the filter model and
the milestone rollup — where a second consumer could not reach it. The
MCP surface and the cross-database ready page both need it.

Move it to a new root package beads/ that depends on browse/ and the
standard library only: rows in, view model out, no net/http, no
html/template, no core. The BrowseSession seam is declared consumer-side
there and names the one method the projections call, so web's larger
BrowseSession satisfies it structurally and a session passes straight
through.

web/beads.go and web/milestones.go keep only their View adapters — slug,
label, template, Applies, and the hand-off of ref and query. The
templates are unchanged: the moved types keep their names and their
display methods, so every dot still resolves.

Pure move plus the beadsMax -> beads.Max export rename. The projection
tests move with the code and become testify; the render tests stay in
web/ unchanged. Same 63 tests pass before and after.

feat(web/beads): hierarchy in the milestone view

Arrange each milestone as a shallow hierarchy instead of a flat list.
- Put the milestone-typed issue(s) on top as the milestone's heads.
- Nest tasks under their epics via parent-child dependency edges,
  with a done/total rollup on the epic row; membership stays label-based.
- Keep remaining members as a loose tail; blocks edges are ignored.
- Read the dependencies table optionally, degrading to a flat list.
- Extend fixtures and real-data logging to cover the hierarchy.

feat(web/beads): promote milestones to their own view/tab

Move the milestone rollup off the board and into a dedicated "Milestones" tab
(a registered View, companion to Beads via the same fingerprint). The tab is
richer than the old board panel: per-milestone progress bar, done/open/in-progress
counts, and the issue list under each milestone (open-work first, closed struck
through), each linking to its beads detail. Issues with no milestone label are
summarized in a footnote. A beads DB with no milestone labels still gets the tab,
showing an empty-state hint.

Removes the board's inline milestone panel and the collectMilestones/BeadMilestone
helpers it used; the grouping now lives in the milestones view.