fix(cli): rename global --name to --account to free --name for subcommands

The marmot harness surfaced the bug on first run: amy stripped
`marmot group create --name "Interop-02"` thinking the global
account selector ran into the group's display-name flag. Result:
amy resolved the "Interop-02" account, found no identity.json, and
errored — no group ever got created.

Renames the global account-selector flag to `--account`. The per-
subcommand `--name` flags (`marmot group create --name "Demo"`,
`profile edit --name "Alice"`, `marmot await group --name X`) are
untouched — they're free of the global parser now that it doesn't
claim the same name.

Sweep:
- `Main.kt`: GlobalFlag.NAME → ACCOUNT, long "--account".
- `Config.kt`: DataDir.resolve param renamed nameFlag → accountFlag;
  every error message points the user at --account.
- `UseCommand.kt`: error hint says `amy --account NAME init`.
- All test wrappers + direct $AMY_BIN calls under cli/tests/ swap
  the global `--name X` for `--account X` (subcommand --name kept
  exactly where it appeared).
- README + DEVELOPMENT updated.

cli/tests/headless/helpers.sh

@@ -14,7 +14,7 @@
 #
 # `--json` opts into amy's machine-readable output (the harness parses it
 # with jq); the default human-text output is for terminal use.
-amy_a() { HOME="$STATE_DIR" "$AMY_BIN" --name alice --secret-backend plaintext --json "$@"; }
+amy_a() { HOME="$STATE_DIR" "$AMY_BIN" --account A --secret-backend plaintext --json "$@"; }
 
 # Run amy, log stderr, surface JSON on stdout, remember last result.
 amy_json() {