fix(cli): close second review on prime-agent sync + switcher (auth, ownership, switching)

Addresses the second round of `config prime-agent` review (10 findings).

auth.json (config/prime-agent.ts) — the settings.json data-loss fix had a twin:
- loadPrimeAgentAuth now fails loudly on corrupt JSON instead of swallow-and-
  rewrite, so one syntax error can no longer destroy the provider API key and
  every other project's credential. hasPrimeAgentAuth shares that guarantee.
- writePrimeAgentAuth writes 0600 (preserving an existing file's mode) instead
  of the default umask — bearer tokens are no longer world-readable on first
  creation.

state ownership (commands/skills.ts) — the ownership model edge cases:
- orphan-removal guard now normalises a canonical scope (project name, or null
  for globals; legacy undefined adopted to current scope) instead of comparing
  null against undefined, so global-only syncs and pre-PR state can no longer
  leave stale skills on disk forever.
- a same-named skill *tracked* to a different project is preserved (with a
  warning) rather than silently overwritten in the shared flat tree.
- mcpServers auto-attach is gated behind !isPrimeAgent with an explicit warning
  (prime-agent's HTTP gateway must not mutate shared mcpd project attachments);
  this also makes the earlier dropped-attach concern explicit rather than silent.

single-active project + switcher (config/prime-agent.ts, prime-agent-extension.ts):
- registerPrimeAgentMcp tags the project's entry mcpctlManaged:true and removes
  other mcpctl-managed entries, so prime-agent has one *active* mcpctl project
  while preserving untagged servers (hand-configured sre, websearch, etc).
- the /mcpctl extension now reads that tag as the single source of truth for the
  active project, fixing the false short-circuit / no-op switch.

config.ts command:
- an explicit -p now updates a differing up-tree .mcpctl-project marker (scope
  no longer silently reverts on the next sync), no-ops when it matches, and
  still never scopes $HOME.
- a project left with no usable credential now exits non-zero (the /mcpctl
  extension checks child exit status, so it no longer reports a successful
  switch after provisioning failed).
- skills sync is treated as best-effort: settings+auth determine switch success,
  so a skills error no longer falsely fails the switch.
- token minting now revokes prior active `prime-agent` tokens before creating a
  fresh one (no more never-expiring token litter / lost-credential duplication).

Tests (544 green): corrupt auth.json refusal, 0600 mode, mint-failure exit
code, single-active dedup preserving untagged sre, cross-project overwrite
preservation, and global-orphan removal on global-only sync.

src/cli/tests/utils/prime-agent-skills.test.ts

@@ -183,4 +183,46 @@ describe('runPrimeAgentSkillsSync', () => {
     expect(result2.removed).toContain('old-skill');
     expect(existsSync(join(installRoot, 'old-skill'))).toBe(false);
   });
+
+  it('does not overwrite a same-named skill owned by a different project', async () => {
+    // Project A installs skill X.
+    const av = [
+      { id: 'a-1', name: 'x-skill', description: 'd', semver: '1.0.0', contentHash: 'sha256:a', metadata: {}, scope: 'project' },
+    ];
+    const af = { 'a-1': { id: 'a-1', name: 'x-skill', description: 'd', semver: '1.0.0', contentHash: 'sha256:a', content: '# version-a\n', files: {} } };
+    const clientA = mockClient({ visible: av, full: af });
+    await runPrimeAgentSkillsSync({ project: 'projA', installRoot, statePath }, deps(clientA));
+    expect(readFileSync(join(installRoot, 'x-skill', 'SKILL.md'), 'utf-8')).toBe('# version-a\n');
+
+    // Project B also has a skill named X with different content.
+    const bv = [
+      { id: 'b-1', name: 'x-skill', description: 'd', semver: '1.0.0', contentHash: 'sha256:b', metadata: {}, scope: 'project' },
+    ];
+    const bf = { 'b-1': { id: 'b-1', name: 'x-skill', description: 'd', semver: '1.0.0', contentHash: 'sha256:b', content: '# version-b\n', files: {} } };
+    const clientB = mockClient({ visible: bv, full: bf });
+    const resultB = await runPrimeAgentSkillsSync({ project: 'projB', installRoot, statePath }, deps(clientB));
+
+    expect(resultB.preserved).toContain('x-skill');
+    // A's version is untouched (not clobbered by B's).
+    expect(readFileSync(join(installRoot, 'x-skill', 'SKILL.md'), 'utf-8')).toBe('# version-a\n');
+  });
+
+  it('removes global orphans on a global-only sync', async () => {
+    // First sync a global skill.
+    const v = [
+      { id: 'g-1', name: 'gone-global', description: 'd', semver: '1.0.0', contentHash: 'sha256:g', metadata: {}, scope: 'global' },
+    ];
+    const f = { 'g-1': { id: 'g-1', name: 'gone-global', description: 'd', semver: '1.0.0', contentHash: 'sha256:g', content: '# g\n', files: {} } };
+    const client1 = mockClient({ visible: v, full: f });
+    const empty = join(tmpDir, 'empty2');
+    mkdirSync(empty, { recursive: true });
+    await runPrimeAgentSkillsSync({ cwd: empty, installRoot, statePath }, deps(client1));
+    expect(existsSync(join(installRoot, 'gone-global'))).toBe(true);
+
+    // Next global-only sync: the global is gone from the visible set.
+    const client2 = mockClient({ visible: [], full: {} });
+    const result2 = await runPrimeAgentSkillsSync({ cwd: empty, installRoot, statePath }, deps(client2));
+    expect(result2.removed).toContain('gone-global');
+    expect(existsSync(join(installRoot, 'gone-global'))).toBe(false);
+  });
 });