feat: Notion backend for the catalog + set command + ls/source/save UX

Persist the entry catalog to a Notion page instead of an external script.

- `hel store notion:<ref>` reads the dump from stdin and writes it into a
  single code block on the page; `hel load notion:<ref>` prints it back.
  `<ref>` is a page id (UUID) or a page title (case-insensitive search).
  Token from HEL_NOTION_TOKEN. Networking lives in helcli (ureq + serde_json);
  the core lib and the wasm build are untouched. argv is dispatched before the
  REPL starts, so the subcommands never read ~/.helrc or prompt for a master.

- `set <key> <value>`: runtime config (e.g. hel_dump, hel_notion_token) from
  ~/.helrc instead of env vars. The map is exported (uppercased) into spawned
  pipe/source children, so `set hel_notion_token ...` reaches `hel store`.
  Values are redacted in history.

- `ls` now matches case-insensitively.
- `source` echoes `source <target>`.
- `save` prints a `< removed` / `> added` diff against the last load/save
  snapshot so removals are noticed before they persist.

README.md

@@ -1,3 +1,63 @@
 # Hel: password generator and manager
 
 Hel is a password manager which newer stores the passwords but generates them according to the rules from the master password and the given name of the password.
+
+## Commands & behavior
+
+- `ls <regex>` matches **case-insensitively** (name, full line, and comment). Add
+  an inline `(?-i)` to the pattern to force case-sensitive.
+- `source <file|command|>` echoes `source <target>` so you see what was loaded.
+- `save` prints a diff against the previously loaded/saved snapshot before
+  persisting: `< line` = removed, `> line` = added — so an accidental removal is
+  visible on the save that would persist it. (Set-based, so dump ordering is
+  irrelevant.) The baseline is seeded on startup load and refreshed on each save.
+
+## Persistence
+
+Hel never stores secrets — only the entry catalog (names + generation rules),
+which is a script of `add …` lines. That script can be saved to / loaded from a
+file, an external command, or a **Notion** page.
+
+### `set`: configuration from `~/.helrc`
+
+Instead of environment variables, settings can live in the init script via
+`set <key> <value>`:
+
+```
+set hel_dump |hel store notion:somenote
+set hel_notion_token secret_xxxxxxxx
+source hel load notion:somenote|
+```
+
+- Keys are case-insensitive. A `set` is never written to the shell history, and
+  its value is redacted in any echo (it may be a secret token).
+- Lookups check the `set` map first, then the matching `UPPERCASE` environment
+  variable. So `set hel_dump …` overrides `$HEL_DUMP`, etc.
+- When hel runs a pipe/source command, the whole `set` map is exported (uppercased)
+  into that child process — that is how `set hel_notion_token …` reaches a spawned
+  `hel store` / `hel load`.
+
+### Notion backend
+
+The `hel` binary doubles as the Notion backend through two non-interactive
+subcommands, designed to drop into the existing pipe plumbing:
+
+- `hel store notion:<ref>` — read the dump from **stdin** and write it into the
+  Notion page (one code block, replaced in place).
+- `hel load notion:<ref>` — fetch the page and print the dump to **stdout**.
+
+`<ref>` is either a Notion page id (UUID) or a page **title** (resolved by search).
+The integration token comes from `HEL_NOTION_TOKEN` (or `set hel_notion_token …`),
+and the target page must be shared with that integration.
+
+Wire it up so `save` pushes to Notion and startup pulls from it:
+
+```
+# ~/.helrc
+set hel_notion_token secret_xxxxxxxx
+set hel_dump |hel store notion:somenote
+source hel load notion:somenote|
+```
+
+(Equivalently, `export HEL_DUMP="|hel store notion:somenote"` and
+`export HEL_NOTION_TOKEN=…` in the shell.)