feat(ssh): wave 5b — the ssh package (remote ExecBackend over ssh2)

Wave 5b of transparent SSH support. NEW standard extension @dispatch/ssh makes
remote execution actually work over SSH, transparently. ssh2 verified to run under
Bun (load-bearing decision #1 confirmed: connects to local sshd :22 + execs).

- config.ts: ~/.ssh/config reader via ssh-config -> Computer[]/ComputerEntry[]
  (read-only discovery; resolves hostName/port/user/identityFile/knownHost).
- hostkey.ts: known_hosts auto-trust-and-pin (present->verify/reject-on-mismatch,
  absent->accept+append; the accept-new analog).
- errors.ts: pure ssh2/SFTP -> node:fs-style .code error mapping (so tools'
  existing ENOENT branches work unchanged).
- pool.ts: SshConnectionPool (per-alias ssh2.Client, lazy connect, keep-alive,
  idle reap ~15m); key-only auth from ~/.ssh (config IdentityFile or default
  id_ed25519/id_rsa); no agent-forwarding, no PTY.
- backend.ts: SshExecBackend implements ExecBackend (spawn via client.exec with
  shell-quoted cwd; fs via SFTP).
- service.ts + extension.ts: activate provides BOTH handles the other units
  consume — remoteExecBackendFactoryHandle (exec-backend: computerId->SshExecBackend)
  AND computerServiceHandle (transport-http: listComputers/getComputer/getStatus/test).
- orchestrator: added packages/ssh to root tsconfig.json refs + bun install.

Tests: 45 pass + 6 sshd-integration skipped (it.skipIf(!process.env.SSH_TEST_HOST)).
Verified: tsc -b EXIT 0, biome clean, 1690 vitest pass (was 1641, +49).
CRs for wave 5c: host-bin registration; CR-5 transport-http barrel re-export;
CR-6 usageCount wiring (deferred-ok, defaults to 0).
Refs: notes/ssh-support-plan.md (decisions §0.5/§13). No merge or push.

1 files changed, 90 insertions, 0 deletions

diff --git a/packages/ssh/src/errors.test.ts b/packages/ssh/src/errors.test.ts
new file mode 100644
index 0000000..234fa49
--- /dev/null
+++ b/packages/ssh/src/errors.test.ts
@@ -0,0 +1,90 @@
+import { describe, expect, it } from "vitest";
+import { type FsError, fsError, mapSshError, sftpStatusToErrno } from "./errors.js";
+
+describe("sftpStatusToErrno", () => {
+	it("maps SSH_FX_NO_SUCH_FILE (3) → ENOENT", () => {
+		expect(sftpStatusToErrno(3)).toBe("ENOENT");
+	});
+
+	it("maps SSH_FX_PERMISSION_DENIED (4) → EACCES", () => {
+		expect(sftpStatusToErrno(4)).toBe("EACCES");
+	});
+
+	it("maps SSH_FX_FILE_ALREADY_EXISTS (11) → EEXIST", () => {
+		expect(sftpStatusToErrno(11)).toBe("EEXIST");
+	});
+
+	it("maps SSH_FX_NOT_A_DIRECTORY (20) → ENOTDIR", () => {
+		expect(sftpStatusToErrno(20)).toBe("ENOTDIR");
+	});
+
+	it("returns undefined for codes with no errno analog", () => {
+		expect(sftpStatusToErrno(1)).toBeUndefined(); // SSH_FX_EOF
+		expect(sftpStatusToErrno(999)).toBeUndefined();
+	});
+});
+
+describe("fsError", () => {
+	it("builds an Error carrying a .code string", () => {
+		const err: FsError = fsError("ENOENT", "no such file: /x");
+		expect(err).toBeInstanceOf(Error);
+		expect(err.code).toBe("ENOENT");
+		expect(err.message).toBe("no such file: /x");
+	});
+});
+
+describe("mapSshError", () => {
+	it("maps a numeric SFTP status code on .code → ENOENT", () => {
+		const err = mapSshError(Object.assign(new Error("fail"), { code: 3 }), "readFile /x");
+		expect(err.code).toBe("ENOENT");
+		expect(err.message).toContain("readFile /x");
+		expect(err.message).toContain("fail");
+	});
+
+	it("maps an SFTP_* string code → ENOENT", () => {
+		const err = mapSshError(
+			Object.assign(new Error("nope"), { code: "SFTP_STATUS_NO_SUCH_FILE" }),
+			"stat /y",
+		);
+		expect(err.code).toBe("ENOENT");
+	});
+
+	it("maps an SFTP permission-denied string → EACCES", () => {
+		const err = mapSshError(
+			Object.assign(new Error("denied"), { code: "SFTP_STATUS_PERMISSION_DENIED" }),
+			"readFile /y",
+		);
+		expect(err.code).toBe("EACCES");
+	});
+
+	it("falls back to message-text sniffing when .code is absent (No such file)", () => {
+		const err = mapSshError(new Error("No such file or directory"), "readFile /z");
+		expect(err.code).toBe("ENOENT");
+	});
+
+	it("falls back to message-text sniffing for permission denied", () => {
+		const err = mapSshError(new Error("Permission denied"), "writeFile /z");
+		expect(err.code).toBe("EACCES");
+	});
+
+	it("surfaces HOST KEY CHANGED as EHOSTUNREACH", () => {
+		const err = mapSshError(new Error("HOST KEY CHANGED for localhost"), "connect");
+		expect(err.code).toBe("EHOSTUNREACH");
+	});
+
+	it("defaults unrecognized errors to EIO", () => {
+		const err = mapSshError(new Error("something weird happened"), "readdir /a");
+		expect(err.code).toBe("EIO");
+	});
+
+	it("never throws — maps a non-Error value", () => {
+		const err = mapSshError("just a string", "readdir /a");
+		expect(err.code).toBe("EIO");
+		expect(err.message).toContain("just a string");
+	});
+
+	it("includes the context prefix in the message", () => {
+		const err = mapSshError(new Error("boom"), "writeFile /path/file");
+		expect(err.message).toContain("writeFile /path/file");
+	});
+});