fix(cc-config): stop pinning CLAUDE_CONFIG_DIR when CLAUDE_HOME is unset

runClaude() unconditionally set CLAUDE_CONFIG_DIR to getClaudeHome()
(~/.claude), so the CLI wrote ~/.claude/.claude.json — a file
cc-discovery.getClaudeJsonPath() never reads (it reads ~/.claude.json,
beside ~/.claude/, by design). Every default-install MCP add/remove and
plugin/marketplace mutation was inert: add toasted success and vanished on
refetch, remove 422'd for every server the panel lists.

With CLAUDE_HOME unset (the default), leave CLAUDE_CONFIG_DIR unset too —
the CLI's own default config dir already lines up with what cc-discovery
reads, so alignment comes from doing nothing. Only pin it when the operator
explicitly set CLAUDE_HOME, which is the one case the pin is meant for.
Left a residual, documented at both sites: with CLAUDE_HOME set, reads still
come from $HOME/.claude.json while the CLI writes
<CLAUDE_HOME>/.claude.json — a split predating this task, affecting a
configuration nobody here runs.

New tests pin the env contract directly (no real CLI needed): runClaude
must not set CLAUDE_CONFIG_DIR when CLAUDE_HOME is absent, and must set it
to CLAUDE_HOME's value when present.

server/lib/cc-discovery.js

@@ -31,6 +31,13 @@ function getProjectClaudeDir(cwd) {
 function getClaudeJsonPath() {
   // ~/.claude.json sits beside ~/.claude/, NOT inside it. Resolve from $HOME
   // so a CLAUDE_HOME override doesn't accidentally relocate it.
+  //
+  // Paired with cc-plugin-cli.js's runClaude(): with CLAUDE_HOME unset (the
+  // default), it deliberately leaves CLAUDE_CONFIG_DIR unset too, so the CLI
+  // writes here (its own default). With CLAUDE_HOME set, this read stays
+  // pinned to $HOME/.claude.json while the CLI writes
+  // <CLAUDE_HOME>/.claude.json — a known, unfixed split for that case; see
+  // runClaude's comment.
   return path.join(os.homedir(), ".claude.json");
 }