heph.nvim: RPC client + buffer editing + wiki-links + journal (slice 11a)

The primary surface begins (tech-spec §8): a Neovim plugin that is a thin
client of the local hephd over its unix-socket JSON-RPC.

- node.resolve {title} → Node|null (heph-core Store + dispatch): exact,
  owner-scoped, non-tombstoned alias-then-title match — the same mapping that
  materializes wiki links, so follow-link jumps to the node the stored link
  points at (never fuzzy search). Unit + rpc_socket integration tests.
- heph.nvim/: vim.uv unix-socket JSON-RPC client (blocking call via vim.wait,
  id-demuxed, partial-line buffered, luanil so JSON null → Lua nil; isolated
  Sessions for tests). Buffer-backed nodes (heph://node/<id>, acwrite;
  BufReadCmd→node.get / BufWriteCmd→node.update, whole-buffer body round-trips
  exactly through the CRDT). [[wiki-link]] follow on <CR>. Daily journal.
  :Heph command surface + completion.
- Headless e2e (§9): a self-contained busted-style runner (tests/e2e/runner.lua)
  — no external plugins, no network, deterministic CI exit codes. Specs: journal
  round-trip, follow-link (+ unresolved no-op), link-two-docs/backlink.
  `make -C heph.nvim test` builds hephd and runs it.

Docs: heph-nvim reference card, §14 tracker (11a done; 11b/11c/11d queued),
changelog fragment.

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

heph.nvim/lua/heph/journal.lua

@@ -0,0 +1,19 @@
+--- Daily journal (tech-spec §8). `journal.open_or_create` is idempotent (the
+--- node id is deterministic in (owner, date)), so opening today's note twice is
+--- safe.
+
+local rpc = require("heph.rpc")
+local util = require("heph.util")
+
+local M = {}
+
+--- Open (creating if absent) the journal for `date` (default: today), in a
+--- buffer. Returns the node.
+function M.open(date)
+  date = date or util.iso_today()
+  local node = rpc.call("journal.open_or_create", { date = date })
+  require("heph.node").open(node.id)
+  return node
+end
+
+return M