fix(cli): harden config prime-agent sync + install /mcpctl switcher extension

Addresses a review of the `config prime-agent` feature and adds the in-app
project switcher.

Safety/correctness fixes (prime-agent's shared, hand-editable ~/.prime/agent
tree must never suffer silent data loss):
- config/prime-agent.ts: loadPrimeAgentSettings now fails loudly on corrupt
  JSON instead of swallowing it and rewriting the file (which destroyed every
  non-mcpServers setting). A project's mcpServers entry is merged (keeping
  user-added fields) rather than replaced wholesale. Added writePrimeAgentAuth
  / hasPrimeAgentAuth helpers for auth provisioning.
- skills sync: unified the near-verbatim prime-agent copy into runSkillsSync
  via a `target: 'claude' | 'prime-agent'` option (prime-agent-skills.ts is now
  a thin wrapper). Under the prime-agent target it: preserves untracked
  pre-existing skill dirs on first sync (no more rm -rf of hand-authored `sre`),
  records per-project ownership so configuring a second project never deletes
  the first project's skills, skips Claude-only hooks/postInstall, and keeps
  the mcpServers auto-attach step.
- config.ts: `config prime-agent` now (a) provisions the bearer credential in
  auth.json (--token, existing entry, or auto-mint via POST /api/v1/mcptokens),
  (b) writes the .mcpctl-project marker only when none exists up-tree and never
  from $HOME, and (c) propagates the skills sync exit code so auth failures are
  reported instead of swallowing them.
- skills.ts: `--agent` is validated; an unknown value errors instead of
  silently running the Claude sync.

New feature: `config prime-agent` installs a `/mcpctl` project-switcher
extension into ~/.prime/agent/extensions/ (skip with --skip-extension). It lists
mcpctl projects via `mcpctl get projects -o json`, lets you pick one from the
prime-agent TUI, applies the switch through the CLI, and reloads the session.

Regenerated shell completions. Tests: 538 pass (new coverage for settings
corruption, entry merge, auth provisioning, extension install/skip, marker
$HOME handling, untracked/cross-project skill preservation, --agent validation).

completions/mcpctl.bash

@@ -125,10 +125,10 @@ _mcpctl() {
             COMPREPLY=($(compgen -W "-p --project -o --output --inspect --stdout --skip-skills -h --help" -- "$cur"))
             ;;
           prime-agent)
-            COMPREPLY=($(compgen -W "-p --project -o --output --gateway-url --skip-skills --dry-run -h --help" -- "$cur"))
+            COMPREPLY=($(compgen -W "-p --project -o --output --gateway-url --token --skip-skills --skip-extension --dry-run -h --help" -- "$cur"))
             ;;
           prime-agent-generate)
-            COMPREPLY=($(compgen -W "-p --project -o --output --gateway-url --skip-skills --dry-run -h --help" -- "$cur"))
+            COMPREPLY=($(compgen -W "-p --project -o --output --gateway-url --token --skip-skills --skip-extension --dry-run -h --help" -- "$cur"))
             ;;
           setup)
             COMPREPLY=($(compgen -W "-h --help" -- "$cur"))