Tasks side pane: restore tree rail lines (├/└/│) so containment reads #1660

What

The tasks side column draws no rail lines. Children indent two cells under their parent (strings.Repeat(" ", depth), internal/tui3/planrail.go:234) with no ├/└ connector and no │ stem, so on a multi-level run the family shape does not read, and a child's title floats ~5 cells right of its parent's title with nothing anchoring it.

The connectors were removed in #1494 (e94bb4f6a), which replaced stem tracking (railEntryRows with stems []bool) with bare indentation — the file's own comment says the column's "forest is gone".

Wanted

Restore the tree rails in the side column, matching the shape the manual and the plandb CLI tree (internal/plandb/cli.go:1462) already draw:

• a child row hangs off ├ (or └ when it is the last child), joined to its parent;
• intermediate rows of a multi-level family carry a │ stem;
• titles align under the connector instead of floating.

Constraints

• Rails follow containment only. A dependency never re-parents a row (the law in internal/tui3/c246_tree_test.go:13); blocked rows keep their queued · waits: <task> wording.
• The base 2-space row prefix (internal/tui3/task.go:4267) and the mark/spinner glyph stay; the connector lives in the lead, not in the row text.

Acceptance

• A two-level family in the pane draws └/├ connectors and │ stems for middle rows.
• No dependency-based re-parenting; waits: wording unchanged.
• go test ./internal/tui3/... green with exact-text tests updated to the rail shape.

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Drafted with CodeAF · reviewed and owned by the author