feat(cli+mcplocal): persistent provider disable/enable

Adds two new subcommands on top of v7's provider lifecycle CLI:

  mcpctl provider disable vllm-local   # release GPU + survive restart
  mcpctl provider enable  vllm-local   # clear the flag, ready to chat

Use case: vLLM keeps crashing on engine init. `down` works for "now"
but the next chat triggers a restart; `disable` writes
`disabled: true` into the provider's entry in ~/.mcpctl/config.json
and short-circuits complete()/ensureRunning() until you re-enable.

Implementation:
- LlmProviderEntry / LlmProviderFileEntry: new optional `disabled` field
- ManagedVllmProvider: setDisabled(bool), isDisabled(), gate in
  complete()/ensureRunning(), expose `disabled` in getStatus()
- mcplocal HTTP: POST /llm/providers/:name/{disable,enable} write the
  config file and apply the change live; /start returns 409 when the
  target is disabled instead of silently failing
- Boot: createSingleProvider honors `entry.disabled` so a known-bad
  vLLM doesn't auto-start on the first chat after mcplocal restart
- CLI: `disable` / `enable` subcommands on `mcpctl provider`; status
  output now shows `(disabled)` next to the state

`enable` is live — provider stays in the registry while disabled, so
flipping the flag back is enough; no mcplocal restart needed.

Tests: cli 437/437, mcplocal 731/731.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

completions/mcpctl.bash

@@ -347,7 +347,7 @@ _mcpctl() {
     provider)
       local provider_sub=$(_mcpctl_get_subcmd $subcmd_pos)
       if [[ -z "$provider_sub" ]]; then
-        COMPREPLY=($(compgen -W "status up down help" -- "$cur"))
+        COMPREPLY=($(compgen -W "status up down disable enable help" -- "$cur"))
       else
         case "$provider_sub" in
           status)
@@ -359,6 +359,12 @@ _mcpctl() {
           down)
             COMPREPLY=($(compgen -W "-h --help" -- "$cur"))
             ;;
+          disable)
+            COMPREPLY=($(compgen -W "-h --help" -- "$cur"))
+            ;;
+          enable)
+            COMPREPLY=($(compgen -W "-h --help" -- "$cur"))
+            ;;
           *)
             COMPREPLY=($(compgen -W "-h --help" -- "$cur"))
             ;;