[keyman] phase 3: derive the age recipient, and survive not having one

main.ts asserted the recipient non-null twice — extractAgePublicKey(...)!
— and the type already said null was possible. With no age.key the vault
encrypted to the string "null": execa stringifies it, age exits 1, and on
the generate path that happens *after* ssh-keygen has written a plaintext
private key into tmpDir, so the user is told the operation failed and left
with a key on disk. Now the recipient is resolved once, remembered on
success, and a null prints the remedy (age-keygen -o <path>) and returns
to the menu. list, copy and decrypt still work without one.

extractAgePublicKey now derives the public key with `age-keygen -y`
instead of scraping the `# public key:` comment. The comment is ordinary
text nothing re-checks; verified that rewriting it does not change what
-y reports, so a stale or forged comment silently encrypted the vault to
a recipient nobody holds the private half of.

The comment survives as a fallback for a machine with no age-keygen,
behind a warning that it is unverified — but not when age-keygen runs and
refuses the file. That means age cannot read the identity, and trusting
the comment there would encrypt to a recipient the vault could never
decrypt with.

runTool throws ToolNotFoundError for ENOENT so the two cases can be told
apart. Its own tests move to tool.test.ts, which keeps real processes;
utils.test.ts mocks execa, since the gate cannot require age installed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/keyman/tests/tool.test.ts

@@ -0,0 +1,56 @@
+/**
+ * Tests for runTool.
+ *
+ * These spawn real processes rather than mocking execa. What runTool exists for
+ * is the shape of an execa failure — a mock would assert only what this test
+ * already assumes. It lives apart from utils.test.ts, which mocks execa to test
+ * the callers.
+ */
+
+import { describe, expect, it } from 'vitest';
+import { runTool, ToolNotFoundError } from '../src/keyman.utils.js';
+
+describe('runTool', () => {
+  it('returns stdout on success', async () => {
+    const result = await runTool('node', ['-e', 'process.stdout.write("hi")']);
+
+    expect(result.stdout).toBe('hi');
+  });
+
+  it('passes options through', async () => {
+    const result = await runTool('node', ['-e', 'process.stdout.write(process.env.PROBE ?? "")'], {
+      env: { PROBE: 'from-options' },
+    });
+
+    expect(result.stdout).toBe('from-options');
+  });
+
+  it('reports empty stdout when the output went elsewhere', async () => {
+    const result = await runTool('node', ['-e', 'process.stdout.write("hi")'], {
+      stdout: 'ignore',
+    });
+
+    expect(result.stdout).toBe('');
+  });
+
+  it('turns a missing binary into an instruction rather than an ENOENT', async () => {
+    const failure = runTool('keyman-no-such-binary', []);
+
+    await expect(failure).rejects.toThrow(ToolNotFoundError);
+    await expect(failure).rejects.toThrow(
+      '`keyman-no-such-binary` was not found on PATH. Install it and try again.'
+    );
+  });
+
+  it('surfaces what the binary wrote to stderr', async () => {
+    await expect(
+      runTool('node', ['-e', 'process.stderr.write("no recipient\\n"); process.exit(1)'])
+    ).rejects.toThrow('`node` failed: no recipient');
+  });
+
+  it('falls back to the command summary when stderr is empty', async () => {
+    await expect(runTool('node', ['-e', 'process.exit(3)'])).rejects.toThrow(
+      /`node` failed: .*exit code 3/
+    );
+  });
+});