heph CLI + export

Slice 7 (tech-spec §1, §5, §9).

- Export (heph-core): render each non-tombstoned node to `<kind>/<id>.md`
  with YAML frontmatter (id, kind, title, timestamps, task scalars,
  aliases, outgoing links) + body. One-way snapshot; `Store::export`
  writes the tree; tombstones excluded. Added `export` RPC method and
  Error::Io.
- `heph` CLI (clap): thin client of hephd over the socket — `next`
  (concise ranked rows), `task`, `doc`, `get`, `export`. Never touches
  SQLite directly.

Tests: 3 export render unit + 2 export round-trip integration + 3 CLI
process tests driving the real `heph` binary against a real daemon
(task→next, empty-store message, export writes files). 70 tests green.

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

docs/changelog.d/v1-prototype.feature.md

@@ -6,4 +6,5 @@ Begin the v1 prototype (Phase 1, tech-spec §11.1), built in TDD slices:
 - "What is next?" ranking (§7): pure, clock-injected, two-stage engine — candidacy filter (do-date as a boolean gate only) then a reorderable list of named dimensions (past-late-on → overdue-amount → attention band → FIFO). `late_on` is the sole urgency signal; blue hidden; red always shown. Proptest-checked total order. `Store::next` surfaces it over SQLite.
 - Recurrence — roll-forward in place (§4.4): completing a recurring task resets its checklist to all-unchecked, logs the occurrence, and advances the do-date to the next RRULE instance after now (skipping misses) — completion never carries forward (proptest-checked). Per-task append-only logs (`log-of`) with `log.append`/`log.tail`; `skip` advances without logging.
 - `hephd` daemon, local mode (§3, §6): exclusive file lock (handoff-ready), line-delimited JSON-RPC over a unix socket exposing the node/task/next/links/log methods, with DB work on tokio's blocking pool. Synchronous client for surfaces/CLI. Model types are serde-serializable.
+- `heph` CLI (§1) — a thin client of the daemon: `next`, `task`, `doc`, `get`, `export`. Export materializes the store to a `<kind>/<id>.md` tree with YAML frontmatter + body (§5), one-way, tombstones excluded.
 - CI runs the Rust suite (fmt/clippy/test) via the project build hook.