docs(ci): move CI handoff into docs/ci/handoff (avoid main collision)

main independently wrote its own docs/handoff/20260615-handoff-70.md the
same day, so my global-sequence handoff-70 was an add/add conflict waiting
to merge. Relocate it to docs/ci/handoff/20260615-handoff-ci-01.md (its own
namespace, like docs/ci/adr) + a README index. main's handoff-70 is
untouched; the merge becomes conflict-free.

docs/ci/handoff/20260615-handoff-ci-01.md

@@ -1,15 +1,18 @@
-# Session handoff — 2026-06-15 (70)
+# CI subproject handoff — 2026-06-15 (ci-01)
 
-Seventieth handover. A dedicated infrastructure session, run on a separate
-**`ci`** branch (in a worktree), that built the project's **entire CI/CD
+First handover for the **CI / release subproject** (the `ci` branch). Kept in
+`docs/ci/handoff/`, a namespace separate from the project's global
+`docs/handoff/` session sequence so it can't collide with `main`'s numbering —
+the same split as `docs/ci/adr/`, and needed for the same reason: `main`
+independently wrote its own **handoff-70** this same day (just as it took
+**ADR-0049**), which would have collided.
+
+A dedicated infrastructure session that built the project's **entire CI/CD
 pipeline** on the self-hosted Gitea Actions runner — from nothing to a live
 gate plus a six-target cross-platform release. Net: the **CI** /
 `requirements.md` **TT5** item and **D1**/**D2** are now done; **D3** and a
-couple of TT5 tails remain. Decisions are recorded in a new ADR namespace,
-**`docs/ci/adr/`** (ADR-ci-001/002/003), kept separate from the main integer
-ADR sequence to avoid cross-branch number collisions (the same split the
-website branch uses — and it paid off: `main` independently took ADR-0049 this
-period, which would have collided).
+couple of TT5 tails remain. Decisions are recorded in the sibling ADR namespace
+**`docs/ci/adr/`** (ADR-ci-001/002/003).
 
 ## §1. State at handoff
 

docs/ci/handoff/README.md

@@ -0,0 +1,21 @@
+# CI / Build subproject — session handoffs
+
+Handover notes for the **CI / release pipeline** work (the Gitea Actions
+workflows under `.gitea/`, the nix flake, the release tooling). Kept in their
+own namespace, separate from the project-wide session handoffs in
+[`docs/handoff/`](../../handoff/), so a CI-branch handoff never competes with
+`main`'s global handoff sequence for numbers — the same split the CI ADRs use
+([`docs/ci/adr/`](../adr/README.md)). This is not hypothetical: `main`
+independently wrote a `handoff-70` the same day this subproject's first handoff
+was drafted.
+
+**Numbering.** Files are named `<date>-handoff-ci-<NN>.md` and referenced in
+prose as `handoff-ci-NN`. Assign the next free `NN` from this index.
+
+## Index
+
+- [handoff-ci-01 — the CI/release pipeline build-out](20260615-handoff-ci-01.md)
+  — Gitea Actions gate (clippy + test) + a six-target release (four via
+  `cargo-zigbuild` on a `v*` tag, two macOS via dispatch on a Tart runner), all
+  on a nix flake; decisions in `docs/ci/adr/`. Built on the `ci` branch, merged
+  `main` in, gate green (2488 tests), ready to promote to `main`.