T-125: argv→IpcRequest translator (CLI grammar in Dart)

Second slice of T-99. Pure Dart function that takes the argv tail of
a `clide ...` invocation and returns either an IpcRequest ready to
dispatch or an ArgvError carrying a pre-built userError response.

The grammar — `SUBSYSTEM VERB [pos...] [--flag value] [--flag=val]
[-- passthrough...]` plus the umbrella commands `status`, `tail`,
`version`, `ping` — sits here so the C client (T-126) is a dumb
pipe: it sends argv as JSON and the server runs the translator
before dispatch.

Wire envelope: cmd is `subsystem.verb` (or just `subsystem` for
umbrella commands). Args is a generic envelope —
`positional: [...]`, `flags: {...}`, `passthrough: [...]` — none
required, all omitted when empty so the dispatch surface stays
minimal. Per-command typed schemas land later as each CLI verb
gets wired end-to-end.

Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -25,6 +25,11 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   dispatch through `DaemonDispatcher` (D-72). Foundation for the C
   `clide` client (T-126) and MCP (T-130). No client yet — testable
   via `socat - UNIX-CONNECT:$SOCK`.
+- argv→IpcRequest translator (`lib/src/cli/argv_to_request.dart`) —
+  parses `clide SUBSYSTEM VERB [pos...] [--flag] [-- passthrough]` and
+  the umbrella commands (`status`, `tail`, `version`, `ping`) per D-6
+  into the wire envelope. Pure Dart; lets the C client (T-126) stay a
+  dumb pipe (T-99 / T-125).
 - Startup project picker — clide now opens to the welcome screen by
   default instead of auto-opening the last project. A per-row
   "always open this project on launch" checkbox in welcome's RECENT