feat(nvim): inline #hashtags become tags on save (§8.3)

On save, whitespace-prefixed `#hashtags` in a node's body are unioned
into its tag set (via `frontmatter.hashtags` + the existing tag diff), so
you can tag a note by writing `#kitchen` inline. A markdown `# heading`
has a space after the `#`, so it never matches. e2e covers it.

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

heph.nvim/lua/heph/frontmatter.lua

@@ -79,6 +79,35 @@ local function date_to_ms(s)
   return os.time({ year = tonumber(y), month = tonumber(m), day = tonumber(d), hour = 0, min = 0, sec = 0 }) * 1000
 end
 
+--- Whitespace-prefixed inline `#hashtags` in `body`, de-duplicated in order.
+--- A markdown heading (`# Title`, `## foo`) has a space after the `#`, so it
+--- never matches. (Scanned across the whole body, code fences included — a
+--- pragmatic v1 simplification.)
+function M.hashtags(body)
+  local seen, out = {}, {}
+  for tag in (" " .. (body or "")):gmatch("%s#([%w_%-]+)") do
+    if not seen[tag] then
+      seen[tag] = true
+      out[#out + 1] = tag
+    end
+  end
+  return out
+end
+
+--- Union of two scalar lists, order-stable, de-duplicated.
+local function union(a, b)
+  local seen, out = {}, {}
+  for _, list in ipairs({ a or {}, b or {} }) do
+    for _, x in ipairs(list) do
+      if not seen[x] then
+        seen[x] = true
+        out[#out + 1] = x
+      end
+    end
+  end
+  return out
+end
+
 --- Set difference of two scalar lists → (added, removed).
 local function set_diff(old, new)
   local oldset, newset = {}, {}
@@ -104,12 +133,16 @@ end
 
 --- Apply the difference between `canonical` (rendered on read) and the buffer's
 --- `fm` by issuing the matching RPCs. `node_id` is the opened node (title/tags
---- target); task scalars route to the owning task (`fm.task`). Raises on any
---- RPC error (e.g. a mistyped `state` or an unknown `project`).
-function M.apply(node_id, canonical, fm)
+--- target); task scalars route to the owning task (`fm.task`). Inline
+--- `#hashtags` in `body` are unioned into the desired tag set (so they manage
+--- tags too); `fm.tags` is updated to that union so the caller caches it as the
+--- new canonical. Raises on any RPC error (a mistyped `state`, an unknown
+--- `project`).
+function M.apply(node_id, canonical, fm, body)
   canonical = canonical or {}
 
-  -- Tags on the opened node.
+  -- Tags on the opened node = the frontmatter list ∪ inline #hashtags.
+  fm.tags = union(fm.tags, M.hashtags(body))
   local added, removed = set_diff(canonical.tags, fm.tags)
   for _, t in ipairs(added) do
     rpc.call("tag.add", { node_id = node_id, tag = t })