Add forward pointer to ADR 019 in superseded ADRs 011–018

[ooloth]

Context

ADR 019 is the terminal ADR in the task-model chain — it supersedes ADRs 011–018 (fully or partially). The chain is now navigable forward: 019 lists what it supersedes in its "What this supersedes" table. But the reverse pointers are missing: ADRs 011–018 do not link forward to 019, so a developer reading any of those ADRs still concludes a wrong model.

(docs/architecture/task-dispatch.md — the original terminal reference — was deleted as part of the ADR 019 implementation. ADR 019 is the authoritative document now.)

What this tracks

Update the status banner or addendum in each superseded ADR to point to ADR 019:

ADR	What to add
011	Partially superseded by ADR 019 (SQLite task tracking dropped; TUI single-process model kept)
012	Superseded by ADR 019
013	Superseded by ADR 019
014	Partially superseded by ADR 019 (dispatch tick and hub task * CLI dropped; worktree mechanics kept)
015	Unchanged — no forward pointer needed
016	Superseded by ADR 019
017	Superseded for now by ADR 019
018	Superseded for now by ADR 019

Done when

Any ADR in the 011–018 range shows "Superseded by ADR 019" in its status line, linking directly to 019-drop-task-model-filesystem-sessions.md.