feat(groups): in-group hierarchy keys (K/J auto-promote + Shift+arrows indent/outdent)

[AdamiecRadek]

Closes #849.

Summary

Adds explicit in-group hierarchy gestures so the cursor's session can change parent without dropping to the CLI:

• K / J (and Shift+↑ / Shift+↓) now auto-promote a sub-session to top-level when it is the first / last child of its parent. Previously they silently no-op'd at the parent's edge.
• Shift+→ demotes the cursor's top-level session to a sub-session of the previous top-level peer (last child).
• Shift+← promotes the cursor's sub-session back to a top-level peer.

All four shortcuts stay scoped to the current group. Cross-group moves remain on M.

Why

Sub-sessions of different parents are interleaved in the visual flat list, and the only way to move a sub-session out of its parent without dropping to the CLI was agent-deck session unset-parent <id> (or M to go to a different group, which is the wrong tool). K/J at the parent boundary was the one ergonomic gap left after #846.

Behaviour

Cursor location	K / Shift+↑	J / Shift+↓	Shift+→	Shift+←
Sub-session, first child	promote (insert before parent)	swap with next sibling	no-op (already a sub)	promote
Sub-session, last child	swap with prev sibling	promote (insert after parent's block)	no-op	promote
Sub-session, middle	swap with prev sibling	swap with next sibling	no-op	promote
Top-level, mid-group	swap with prev top-level	swap with next top-level	demote under prev peer	no-op
Top-level, first in group	no-op (group edge)	swap with next top-level	no-op (no prev peer)	no-op
Top-level, last in group	swap with prev top-level	no-op (group edge)	demote under prev peer	no-op

Shift+→ (demote) is a no-op when the source has its own children — single-level nesting is preserved (same invariant session set-parent already enforces).

Implementation

• MoveSessionUp / MoveSessionDown extended to call inst.ClearParent() and reposition the slice entry when no same-parent sibling exists in the move direction.
• New GroupTree.PromoteSession(inst) — explicit clear-parent in the same group; same data mutation as session unset-parent.
• New GroupTree.DemoteSession(inst) — set parent to previous top-level peer, insert at that peer's last-child slot. Mirrors session set-parent validation (no nested sub-sessions, no self-parent, no demote when source has children).
• home.go binds shift+left and shift+right to the same path as shift+up / shift+down (rebuild flat items, restore cursor, save).

Tests

internal/session/groups_test.go:

• TestMoveSessionUp_FirstChildPromotes
• TestMoveSessionDown_LastChildPromotes
• TestMoveSessionUp_OnlyChildPromotes, TestMoveSessionDown_OnlyChildPromotes
• TestMoveSessionUp_TopLevelAtFirstNoOp, TestMoveSessionDown_TopLevelAtLastNoOp
• TestPromoteSession_SubSessionBecomesTopLevel, TestPromoteSession_TopLevelNoOp
• TestDemoteSession_TopLevelBecomesLastChild
• TestDemoteSession_FirstTopLevelNoOp
• TestDemoteSession_SubSessionNoOp
• TestDemoteSession_WithChildrenNoOp

The existing TestMoveSessionUp_SubSessionAtFirstSiblingNoOp was replaced — its no-op contract is now an auto-promote.

Docs

• In-app help overlay (?) gets explicit K / Shift+↑ and J / Shift+↓ rows (was a single combined row), plus a new Shift+→/← row. Reorder rows now mention the auto-promote at the parent edge.
• skills/agent-deck/references/tui-reference.md — same direction split + new indent row.
• CHANGELOG.md Unreleased / Added.

Test plan

• go test -count=1 -race -run "TestMoveSession|TestPromoteSession|TestDemoteSession|TestHelpOverlay" ./internal/session/ ./internal/ui/
• go vet ./internal/session/... ./internal/ui/...
• Built locally and verified visually — boundary promote, explicit promote, explicit demote, last-child→demote-into-next-peer, first-top-level no-op, last-top-level no-op, demote with children no-op, help overlay layout at terminal width 120.