Fix review findings, add per-project config and multi-target pushremote

Correctness and security fixes found by a review of the initial commit:

- init decided whether a server repository existed from the *local*
  remote.origin.url, so an unlinked project directory skipped the
  confirmation and rm -rf'd the remote history. It now asks the server,
  and aborts when the server cannot be reached.
- Project names and paths were interpolated unquoted into the remote
  shell command strings: a space split one path into two arguments and a
  backtick executed on the git server. Everything now goes through shq(),
  and chained remote commands use && so a failed cd cannot let the next
  command run in the login directory.
- Bare `cd` panicked with an index-out-of-range and took down the shell;
  it now deselects the project.
- Command-line mode set PRJ to the whole path below BASE, so `mgsh push`
  from a subdirectory staged only that subtree and addressed a bogus
  server path. It now truncates at the first path element.
- `list` hardcoded owner and group "git git" in its regex and silently
  printed nothing on any server where the repositories are owned by
  someone else.
- The config parser kept inline "#" comments in values although the
  README and the example file document them, so `mirror = true # ...`
  silently disabled mirroring.
- ~/.mgshrc holds an API token but was created world-readable.
- The mirror token was passed on git's command line, visible in the
  process table; it now goes through GIT_CONFIG_*.
- tag, count and dist ran without a repository and operated on BASE.
- checkout dropped its git options, because the dispatcher strips -x
  flags from the word list.
- REPO was read with a plain `git config`, inheriting a foreign origin
  from an enclosing repository; it is now local-only and, being dead
  state otherwise, no longer recomputed on every prompt.
- getkey consumed a single byte, leaving the rest of a typed answer in
  the tty queue where readline ran it as a command.
- The REPL spun on any readline error that was neither EOF nor interrupt.
- Tab completion cached an empty repository list after one failed ssh.
- Startup did a blocking DNS lookup and three `git config --global`
  writes on every invocation.

New:

- A project may carry its own .mgshrc, overriding the global settings
  while it is active. Resolution order is ~/.mgshrc -> <project>/.mgshrc
  -> MGSH_*; base and the git identity keys stay global. It is read when
  the project changes, and `rescan` reloads it.
- pushremote mirrors to any number of servers, configured as
  remote.<name>.url/key/type/visibility blocks. `pushremote` pushes to
  all of them, `pushremote @name ...` to a selection, and `remotes = ...`
  restricts and orders the set. Each target owns a git remote of the same
  name; a failing target no longer stops the others.
- `config` shows the resolved configuration, its sources and the mirror
  targets with masked tokens; `config -k` lists the setting names.
- gitkey was parsed and documented but never used. It is now the ssh
  identity for the git server, for mgsh's own ssh calls and, via
  GIT_SSH_COMMAND, for the git commands mgsh runs.
- config, count, login and cloneall work from the command line too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

alias.go

@@ -47,7 +47,7 @@ var builtinCmds = map[string]bool{
 	"pushremote": true, "overview": true, "archive": true, "init": true,
 	"login": true, "cd": true, "checkout": true, "clone": true, "cloneall": true,
 	"open": true, "view": true, "count": true, "tag": true, "alias": true,
-	"unalias": true,
+	"unalias": true, "config": true,
 }
 
 func isBuiltin(name string) bool { return builtinCmds[name] }
@@ -169,7 +169,9 @@ func saveAliases() {
 	if blk := aliasBlock(aliases); blk != "" {
 		b.WriteString("\n" + blk)
 	}
-	if err := os.WriteFile(path, []byte(b.String()), 0644); err != nil {
+	// os.WriteFile only applies the mode when creating the file, so an existing
+	// ~/.mgshrc keeps whatever the user set; a fresh one is created private.
+	if err := os.WriteFile(path, []byte(b.String()), configMode); err != nil {
 		errorln("could not save aliases: " + err.Error())
 	}
 }