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/task.lua

@@ -76,6 +76,36 @@ function M.skip_current()
   end, "occurrence skipped")
 end
 
+--- Promote the context-item line under the cursor to a committed task; the
+--- daemon rewrites that line into a `[[link]]` to the new task. `opts.attention`
+--- optional. Returns the created task.
+function M.promote_under_cursor(opts)
+  opts = opts or {}
+  local buf = vim.api.nvim_get_current_buf()
+  local container_id = vim.b[buf].heph_node_id
+  if not container_id then
+    util.notify("not in a heph node buffer", vim.log.levels.WARN)
+    return nil
+  end
+  -- Save first so the daemon's body matches what the user sees on screen.
+  if vim.bo[buf].modified then
+    require("heph.node").write(buf, vim.api.nvim_buf_get_name(buf))
+  end
+  local item_ref = util.context_item_index_at_cursor(buf)
+  if not item_ref then
+    util.notify("cursor is not on a context item (- [ ])", vim.log.levels.WARN)
+    return nil
+  end
+  local task = rpc.call("task.promote", {
+    container_id = container_id,
+    item_ref = item_ref,
+    attention = opts.attention,
+  })
+  require("heph.node").reload(container_id) -- pull the rewritten body
+  util.notify("promoted to a task")
+  return task
+end
+
 --- Append a line to the current task's log (the resumption breadcrumb).
 function M.log_append_current(text)
   if not text or #text == 0 then