doc: Add install guide instructions.

docs/DEVPLAN.md

@@ -275,6 +275,36 @@ navigate directories without spawning new processes.
   query, or the viewport to do a lookup after each filter
   pass.
 
+- **Confirm-with-arguments (Shift+Enter).** Select an item
+  and also pass free-text arguments alongside it. Primary
+  use case: app launcher where you select `ls` and want to
+  pass `-la` to it. The output would include both the
+  selected item and the user-supplied arguments.
+
+  Open questions:
+  - UX flow: does the filter text become the args on
+    Shift+Enter? Or does Shift+Enter open a second input
+    field for args after selection? The filter-as-args
+    approach is simpler but conflates filtering and
+    argument input. A two-step flow (select, then type
+    args) is cleaner but adds a mode.
+  - Output format: separate field in the JSON output
+    (`"args": "-la"`)? Second line on stdout? Appended to
+    the label? Needs to be unambiguous for scripts.
+  - Should regular Enter with a non-empty filter that
+    matches exactly one item just confirm that item (current
+    behaviour), or should it also treat any "extra" text
+    as args? Probably not, too implicit.
+  - Keybind: Shift+Enter is natural, but some terminals
+    don't distinguish it from Enter. May need a fallback
+    like Ctrl+Enter or a normal-mode keybind.
+
+  This is a core feature (new keybind, new output field),
+  not just a launcher script concern. Fits naturally after
+  phase 4 (multi-select) since it's another selection
+  mode variant. The launcher script would assemble
+  `{selected} {args}` for execution.
+
 ## Future Ideas (Unscheduled)
 
 These are things we've talked about or thought of. No
@@ -306,3 +336,11 @@ commitment, no order.
   (optionally into a tmux session). Needs GUI frontend
   (phase 8) and frecency sorting.
   See `docs/use-cases/app-launcher.md`.
+  Setup guides: `docs/guides/app-launcher.md`.
+- App description indexing: a tool or subcommand that
+  builds a local cache of binary descriptions from man
+  pages (`whatis`), .desktop file Comment fields, and
+  macOS Info.plist data. Solves the "whatis is too slow
+  to run per-keystroke" problem for the app launcher.
+  Could be a `pikl index` subcommand or a standalone
+  helper script.