The time command drops argument quoting, so it cannot time any command with a quoted argument

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DISCLOSURE: LLM-GENERATED TEXT

Problem

time re-joins its arguments with single spaces and re-tokenizes, so
a quoted argument is destroyed. time set-variable greet "two words"
re-enters CommandShell.RunCommand as three tokens and is rejected:
set-variable requires exactly 2 arguments.

Evidence

• Runtime/CommandTerminal/Backend/BuiltinCommands.cs:375 -
  shell.RunCommand(JoinArguments(args)).
• Runtime/CommandTerminal/Backend/BuiltinCommands.cs:636 -
  JoinArguments emits args[i].contents joined with ' ', dropping
  CommandArg.startQuote / endQuote.
• Runtime/CommandTerminal/Backend/CommandShell.cs:974 - the joined
  string goes back through CommandTokenizer.Tokenize.
• BuiltinCommands.cs:514 - set-variable is
  MinArgCount = 2, MaxArgCount = 2, so the extra token is a hard
  error, not a silently wrong value.

time help works, and only because help takes no arguments.

Why it is not one line

log-terminal and log use the same JoinArguments, and there the
developer wants the flattening: log "a b" should print a b, not
"a b". So the quoting has to be conditional on the caller, which
means either a second join method or a flag on the existing one.

The better long-term shape is for time not to re-tokenize at all - it
already has the parsed CommandArg[] and throws it away. That needs a
CommandShell entry point that takes pre-parsed arguments, which is a
larger change; decide which you want.

The same care applies to the quoting style. CommandTokenizer accepts
both "..." and '...', and escapes need to match what the tokenizer
unescapes, so the quoting and the escaping belong next to each other
rather than duplicated in a command handler.

Acceptance

time <anything the console can run without time>` produces the same
result, error messages included, and the same timing line.

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DISCLOSURE: LLM-GENERATED TEXT

Landed in #201

time dispatches the parsed CommandArg[] it already holds instead of
rejoining contents with single spaces and re-tokenizing.

The issue offered two shapes and asked which one was wanted. The better one
turned out to already exist: CommandShell.RunCommand(string, CommandArg[])
takes pre-parsed arguments, so nothing has to round-trip through text and the
conditional quoting the issue anticipated is not needed - log and
log-terminal keep the flattening they want, and time no longer flattens at
all.

One behaviour did change, deliberately: $variable is now substituted once
rather than twice. The old path flattened to a string and re-tokenized it, so
a stored value that was itself a variable reference was expanded again. That
is what time X running X means, and it is in the changelog.