heph.nvim: context-item promotion + Dagger headless-nvim CI (slice 11c)

Backend (TDD):
- task.promote {container_id, item_ref, attention?, project?}: mint a committed
  task from the item_ref-th `- [ ]` context item (1-based, document order via a
  new extract::context_item_lines) and rewrite that source line into a [[link]]
  to it. Unit + rpc_socket tests.
- resolve_id now excludes canonical-context docs, so [[Task Title]] resolves to
  the task, not its identically-titled context doc (deterministic; a general fix
  surfaced by promotion's ULID-tiebreak ambiguity).

Plugin: :Heph promote / promote_under_cursor (save-if-dirty → compute item index
with a code-fence-aware scanner mirroring extract.rs → task.promote → reload the
rewritten buffer). e2e spec (f): promote a context line, assert the new task in
next, the source line became a link, and the container backlinks the task.

CI via Dagger: a test_nvim function bakes a pinned, arch-detected Neovim
(v0.11.2 — Debian's is too old for vim.uv) onto rust:1-bookworm, builds hephd,
and runs the self-contained shim suite (cargo + target cache volumes);
build.yaml calls `dagger call test-nvim`. run.lua now fails on zero specs (no
false-green). Validated end-to-end: passing suite → exit 0, failing spec →
Dagger exit 1.

117 Rust tests + 7 nvim e2e specs green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

heph.nvim/lua/heph/util.lua

@@ -18,6 +18,31 @@ function M.parse_uri(uri)
   return uri:match("^heph://([^/]+)/(.+)$")
 end
 
+--- The 1-based index of the context item on the cursor's line among the
+--- buffer's context items (document order, fenced code skipped) — the `item_ref`
+--- for `task.promote`. Mirrors heph-core's `extract.rs` ordering. Returns nil if
+--- the cursor line is not a `- [ ]` / `- [x]` item.
+function M.context_item_index_at_cursor(buf)
+  local cur = vim.api.nvim_win_get_cursor(0)[1]
+  local lines = vim.api.nvim_buf_get_lines(buf, 0, cur, false) -- lines 1..cursor
+  local in_fence, count, last_is_item = false, 0, false
+  for _, line in ipairs(lines) do
+    if line:match("^%s*```") then
+      in_fence = not in_fence
+      last_is_item = false
+    elseif not in_fence and line:match("^%s*[-*+]%s+%[[ xX]%]") then
+      count = count + 1
+      last_is_item = true
+    else
+      last_is_item = false
+    end
+  end
+  if last_is_item then
+    return count
+  end
+  return nil
+end
+
 --- Notify with a consistent `heph:` prefix.
 function M.notify(msg, level)
   vim.notify("heph: " .. msg, level or vim.log.levels.INFO)